DE102021004361A1 - Antimicrobial and photocatalytic glaze composition - Google Patents

Abstract

The invention relates to a glaze composition with an antimicrobial and photocatalytic effect for the surface finishing of a ceramic object, which comprises at least one silicon compound, at least one aluminum compound, at least one titanium compound and at least one antimicrobial metal compound, the metal of the at least one antimicrobial metal compound being selected from the group consisting of made of zinc, tin, copper, calcium, strontium, lanthanum, cerium, iron, nickel, vanadium, molybdenum and tungsten. In the glaze composition, the weight ratio of aluminum to silicon is greater than 0.3, the proportion of the metal of the at least one antimicrobial metal compound in the glaze composition is at most 20% by weight, and an aqueous slurry of the glaze composition has a pH value that is not neutral area.
The invention further relates to a method for coating a ceramic object with the antimicrobial and photocatalytic glaze composition and a coated ceramic object produced according to the method.

Description

The invention relates to a glaze composition which can be used to finish the surface of a ceramic article and impart antimicrobial and photocatalytic properties to the surface. The ceramic object can be, for example, sanitary ceramics, building ceramics or laboratory ceramics, for example a roof tile, a tile, a slab - i.e. a large plate with an edge length of at least 1.5 m, which is used as such or in smaller ones can be divided into pieces - a facade panel, fireclay, a chimney pipe, a kitchen worktop, a kitchen sink or the like. Surface finishing can be carried out on fired, partially fired or unfired ceramic substrates such as green ceramic bodies, clay or loam, as well as engobed, glazed or unglazed substrates. The glaze composition is particularly suitable for the surface finishing of fine stoneware. Since this is mostly not glazed, its surface has a high sensitivity to dirt, which should be reduced with the help of a surface treatment, also known as surface refinement.

Known surface finishes usually consist of a mixture of different metal oxides which, according to the Seger formula, are present in such proportions that the overall mixture has a neutral pH. In order to obtain the desired antimicrobial properties, at least one metal compound with an antimicrobial effect, such as zinc oxide, is added to the metal oxide mixture. Such a composition and the corresponding surface finish are, for example, in EP 3 053 902 A1 described. A key feature of this and similar metal oxide compositions is a very high proportion of zinc oxide, exceeding 35 percent by weight in the case of EP '902. Investigations of such compositions by the applicant have shown, however, that the corresponding surface finishes, despite the high proportion of zinc oxide, have only very low antimicrobial properties. In addition, the high proportion of zinc oxide means that only matt glazes are available, some of which are rough because the zinc oxide forms coarser particles on the surface.

On the other hand, surface finishes that have pronounced antimicrobial and photocatalytic properties, can be applied very thinly and do not change the optical properties of the substrate surface to be finished are desirable. The object of the invention is accordingly to specify a glaze composition which gives a ceramic object treated therewith a surface finish with pronounced antimicrobial and photocatalytic properties without noticeably changing the optical properties of the ceramic object.

This object is achieved with the glaze composition according to claim 1 and the ceramic article according to claim 14 obtainable by the method according to claim 13, which also forms part of the present invention. Preferred developments are described in the respective dependent claims.

In a first aspect, the invention thus relates to a glaze composition with an antimicrobial and photocatalytic effect for the surface finishing of a ceramic object, which comprises at least one silicon compound, at least one aluminum compound, at least one titanium compound and at least one antimicrobial metal compound, the metal of the at least one antimicrobial metal compound being selected from the group consisting of zinc, tin, copper, calcium, strontium, lanthanum, cerium, iron, nickel, vanadium, molybdenum and tungsten. The weight ratio of aluminum to silicon in the glaze composition is greater than 0.3, and the proportion of the metal of the at least one antimicrobial metal compound in the glaze composition is at most 20% by weight. In addition, the pH of an aqueous slurry of the glaze composition is in a non-neutral range, i.e. it is either acidic (pH <7) or alkaline (pH >7).

In the context of the invention, antimicrobial properties are understood to mean, in particular, antibacterial, antiviral, fungicidal and algicidal properties. In the present case, an antimicrobial metal compound or an antimicrobial metal refers to substances that are suitable for imparting antimicrobial properties to a surface finish produced using them. A photocatalytic effect is usually understood to mean influencing the kinetics of a chemical reaction under the influence of light, for example in order to start the chemical reaction, accelerate it and/or direct it in a specific direction. In the context of the invention, the photocatalytic action is mainly aimed at breaking down pollutants and/or pollutants, in particular air pollutants. According to the invention under a photocatalytic We kung to understand a catalytic effect that takes place under the influence of light, and in particular the decomposition of pollution and / or (air) pollutants under the influence of light using at least one photocatalytic metal compound or a photocatalytic metal. The latter refer to substances that are able to achieve a photocatalytic effect. Photocatalytic effects can be detected in a manner known in the prior art, for example by means of a methylene blue test. In the following, for the sake of simplicity, the antimicrobial metal compound is also simply referred to as “metal compound”. The antimicrobial and photocatalytic properties are summarized under "active properties".

In the prior art, it has hitherto been customary to put together a glaze composition from three groups of metal oxides—the neutral, the base and the acid side—in accordance with the Seger formula so that the overall composition has a neutral pH. In the investigations on which the present invention is based, however, it has surprisingly been shown that significantly better active properties are achieved if this customary procedure is deviated from and the pH of the glaze composition is deliberately adjusted either in the alkaline or in the alkaline or is shifted to the acidic range. The pH of the aqueous suspension of the glaze composition according to the invention preferably deviates by at least 0.2, preferably at least 0.3 or 0.4 or 0.5 or 0.6 or 0.7, in particular by at least 0.8 or 0.9 and more preferably by at least 1.0 from neutral pH 7. The pH value is therefore in particular less than 6.x or 5.x or 4.x for the acidic range and more than 8.x or 9.x or 10.x for the alkaline range, where x is 0.1 in each case , 2, 3, 4, 5, 6, 7, 8 or 9.

The glaze composition is selected in such a way that the pH value deviating from pH 7 is not only present in its aqueous suspension, but also remains either in the acidic or alkaline range after the composition has been fired and, accordingly, the surface of the glaze composition refined with it ceramic article has a pH value as described above which is not neutral. The pH of the finished surface can be determined, for example, using an aqueous bromothymol blue solution, which is suitably applied to the finished surface of the ceramic article. The bromothymol blue solution is known to be colored green at neutral pH, yellow in the acidic range, and blue in the alkaline range.

Of particular importance for the development of the positive properties of the glaze composition according to the invention is not only the pH, but also the weight ratio of aluminum to silicon, which is present in the form of at least one aluminum compound and at least one silicon compound in the glaze composition according to the invention. In contrast to what is customary in the prior art, a weight ratio based on the metal atoms is defined according to the invention, since the extensive investigations carried out within the scope of the invention have shown that to achieve a good antimicrobial and photocatalytic effect on the production of suitable mixed crystal structures on the surface of the finishing layer, which in turn requires a certain ratio of aluminum to silicon. This weight ratio of aluminum to silicon in the glaze composition is greater than 0.3. This weight ratio is generally in excess of that found in conventional glaze compositions for making surface finishes. That is, the proportion of aluminum relative to silicon is generally higher in the glaze compositions of the present invention than comparable prior art glaze compositions. Thus, starting from a known glaze composition, this can be modified to a glaze composition according to the invention by adding an aluminum compound and optionally further compounds for adjusting the pH value and/or an antimicrobial metal compound. The weight ratio of aluminum to silicon in the glaze composition according to the invention is preferably in the range from 0.3 to 5.0, preferably in the range from 0.3 to 4.0 or 0.3 to 3.0 or 0.3 to 2.0 , particularly preferably at 0.4 to 1.2 or 0.5 to 1.2 or 0.6 to 1.2 or 0.7 to 1.2 and in particular at 0.8 to 1.2.

On the other hand, the proportion of the antimicrobial metal compound in the glaze composition according to the invention is comparatively small. It is added so that the metal content of the antimicrobial metal compound is at most 20% by weight based on the total glaze composition. The calculation of the proportions by weight of the individual components of the glaze composition and accordingly the total weight of the glaze composition is carried out without taking any solvents into account. Of course, this does not rule out that individual, several or even all components of the glaze composition are added in dissolved or suspended form. For the calculation In this case, however, the proportion of the solvent in which the added component is dissolved or suspended is not included in the calculation of the proportion by weight or the proportion of solvent is deducted from the calculation of the proportion by weight of the corresponding component.

The comparatively small proportion of at least one antimicrobial metal compound, hereinafter also referred to simply as metal compound, which is required to achieve the antimicrobial effect is not only advantageous in terms of reduced costs, but actually usually also a prerequisite for that at all a sufficient effect is achieved. The investigations accompanying the invention surprisingly showed that precisely the high proportion of antimicrobial metal compounds used in the prior art means that the surface treated with a corresponding glaze composition has a comparatively low antimicrobial effect. The investigations carried out suggest that the metal compounds used, when used in larger amounts, act as fluxing agents in the glaze composition. As a result, they melt together with the other components of the glaze composition and become embedded in the vitreous melts that form. As a result, however, they are no longer present as active components on the surface of the coating formed and lose their antimicrobial activity almost completely. According to the investigations carried out, this effect practically always occurs when the proportion of the metal of the antimicrobial metal compound in the glaze composition is over 20 percent by weight. Accordingly, in the glaze composition of the present invention, the antimicrobial metal compound is added so that the proportion of the antimicrobial metal is at most 20% by weight of the glaze composition. However, the proportion of the metal in the antimicrobial metal compound is preferably lower and is, for example, at most 19, 18, 17, 16, 15, 14, 13, 12, 11 or 10 percent by weight, particularly preferably less than 9 or 8 percent by weight and in particular less than 7 or 6 or 5% by weight of the total glaze composition. If several antimicrobial metal compounds are used, the amounts given relate to the totality of the metals of all antimicrobial metal compounds.

It is believed that the reason why extremely high proportions of an antimicrobial metal compound such as zinc oxide are used in the prior art is that the proportion has been continuously increased due to the incorporation of the metal compound into a vitreous melt to achieve a slight antimicrobial effect, so that the proportion of zinc oxide ended up being over 35 percent by weight. These extremely high proportions of zinc oxide then no longer melt completely, so that zinc oxide particles remain on the surface of the coating. However, this leads to the already mentioned disadvantages of a cloudy and possibly rough surface, the activity of which is still comparatively low. If a zinc compound such as zinc oxide is used in the glaze composition according to the invention, its proportion is preferably at most 10 percent by weight, particularly preferably less than 8 percent by weight and in particular 3 to 7 percent by weight of the entire glaze composition.

The inventive combination of the features of a suitable ratio of aluminum to silicon, the adjustment of the pH so that it is not neutral, and a suitable and comparatively small amount of an antimicrobial metal compound in the glaze composition has the result that the metal compound during firing the glaze composition does not lose its antimicrobial activity but is present in an active form on the surface of the finishing layer formed on the ceramic article. It is essential that the metal compound does not form a vitreous melt with other components of the glaze composition and is not embedded in such a melt. Rather, mixed crystals are formed on the surface of the finishing layer, in the crystal lattice of which metal atoms of the at least one antimicrobial metal compound are incorporated. This mixed crystal formation is only possible if the parameters described above are observed at the same time. Neither the high proportion of aluminum in the glaze composition nor the adjustment of the pH value nor the small amount of the antimicrobial metal/antimicrobial metal compound lead to the formation of the desired mixed crystals and surface finishes with the desired optical and active properties. The desired mixed crystal formation can be promoted in a targeted manner by a suitable choice of the components of the glaze composition. The selection is made in particular taking into account the conditions under which the firing process of the glaze composition is to take place on the ceramic object to be finished, in particular taking into account the firing temperature and the firing time. The components and in particular the antimicrobial metal compound are expediently selected in such a way that they form a melt at the selected firing temperature in the specified firing time, from which crystallization and the formation of the desired mixed crystals can take place.

In the prior art, it has hitherto been customary to add metal compounds and in particular also the antimicrobial metal such as zinc in the form of low-melting frits to the glaze composition. In these frits, the antimicrobial metal is embedded in a pre-melted metal-containing glass. If the antimicrobial metal compound is used exclusively in this form, the formation of mixed crystals in surface finishing is hardly possible, and glass-like melts are almost always formed in which the antimicrobial metal compound has completely lost its activity. If the glaze composition then contains other fluxes such as alkali or alkaline earth metal compounds, the formation of mixed crystals is virtually impossible. Accordingly, in the present invention, it is not preferable to add the antimicrobial metal compound in the form of a frit. However, this does not completely rule out the use of frits in the glaze composition according to the invention. Frits can be used, for example, to set a suitable viscosity in the melt of the glaze composition, which enables crystallization of the desired mixed crystals from the melt. In addition, it is possible and sometimes even advantageous to use frits containing at least one antimicrobial metal in the glaze composition, and it has been observed that the antimicrobial effect of the composition can thereby be even further improved. However, such frits are preferably used only in addition to an antimicrobial metal compound that is not present in the form of a frit. For example, frits containing zinc can advantageously be used in combination with at least one zinc-containing compound which is not in the form of a frit in the glaze composition according to the invention. On the other hand, the desired antimicrobial effect cannot usually be achieved with frits containing zinc alone.

The metal of the antimicrobial metal compound used according to the invention is selected from the group consisting of zinc, tin, copper, calcium, strontium, lanthanum, cerium, iron, nickel, vanadium, molybdenum and tungsten. Among these, the tin compounds are those that can be used to produce glazing compositions and surface finishes whose pH is in the acidic range. Tin oxide, tin hydroxide and tin chloride or any mixtures thereof are used as preferred tin compounds. Tin hydroxide and tin chloride are converted into tin oxide during firing and, like tin oxide itself, form oxidic - i.e. oxygen-containing - mixed crystals together with other components of the glaze composition, which accumulate on the surface of the surface finish of a ceramic object treated with the glaze composition and are responsible for a high antimicrobial activity of the provide a refined surface. The other antimicrobial metal compounds - i.e. those of zinc, copper, calcium, strontium, lanthanum, cerium, iron, nickel, vanadium, molybdenum and tungsten - are suitable for the production of basic glaze compositions, which lead to refined surfaces with an alkaline pH value. Among these, compounds of zinc, strontium, iron, copper and nickel are preferred, with zinc compounds being particularly preferred in the present invention. Combinations of the metal compounds mentioned can also be used. Combinations of zinc and strontium compounds, for example, produce very good results. Even tin compounds can be used to form alkaline glaze compositions when combined with a sufficient amount of alkaline components. However, this is not preferred according to the invention, also because tin compounds usually lead to clouding and thus to an optical impairment of the surface finish obtained.

If the pH of the glaze composition containing at least one of these metal compounds is to be in the alkaline range, the metal compounds are preferably selected from the group consisting of oxides, hydroxides, carbonates, nitrates, sulfates, chlorides or mixtures thereof. Sulfates and chlorides are preferably only used in combination with other naturally basic metal compounds in order to be able to safely adjust the pH of the glaze composition to above 7. Metal compounds which are particularly preferred according to the invention are zinc oxide, zinc hydroxide or zinc carbonate and mixtures thereof. As mentioned, these can also be combined with other antimicrobial metal compounds, such as those of strontium, in particular strontium hydroxide and/or strontium carbonate, the latter also giving good results on their own—without zinc compounds. As in the case of the tin compounds, the metal compounds that are not initially present as oxides are converted into oxides during the firing process. In addition to the metal compounds expressly mentioned, it is also possible in principle to use other metal compounds which can be converted into the corresponding oxides during firing, for example oxalates. Oxidic mixed crystals are formed from the oxides in combination with elements of the other components of the glaze composition during the firing process. Here, too, it can be seen that these mixed crystals accumulate on the surface of the finishing layer, where they develop a high antimicrobial effect.

It is also possible to add the components of the glaze composition at least partially in the form of naturally occurring rocks. These are ground up before addition and added in powder form. Carbonate minerals, for example, are suitable for use in a basic pH glaze composition. Rocks that are low in silicate and/or rocks that are rich in aluminum are preferably used to set the desired aluminum-to-silicon ratio, which can be found among other things among igneous rocks. Suitable examples are bentonite, calcite, diabase, dolomite, foil syenite, kaolin, melilite, montmorillonite, mullite, nepheline, olive, plagioclase, pyroxene and syenite. The parameters pH value, Al:Si ratio and proportion of the antimicrobial metal compound specified according to the invention can be adjusted by suitable selection of the minerals and their proportions by weight—if necessary by supplementing missing proportions by adding pure individual compounds.

With regard to the composition of the components of the glaze composition according to the invention, it has proven advantageous to select only those with a basic pH if a glaze composition with a pH in the alkaline range is to be produced, and - apart from the required silicon compound - acidic and in particular to avoid strongly acidic components as far as possible. Conversely, for a glaze composition with an acidic pH, if possible only those components are used which have an acidic pH, while alkaline and in particular strongly alkaline components are not used if possible - apart from the required at least one aluminum compound. In this respect, the invention deviates from the generally customary procedure in which, according to the Seger formula, components from the basic, neutral and acidic groups are put together in such a way that a pH value in the neutral range results. However, according to the investigations carried out, it is precisely this deviation from the usual procedure that leads to the formation of the desired mixed crystals on the surface of the finishing layer and thus to the significantly improved antimicrobial and photocatalytic properties.

The at least one aluminum compound that must be contained in the glaze composition according to the invention is preferably selected from the group consisting of aluminum oxide, aluminum hydroxide, aluminum nitrate and aluminum alcoholates. The latter can be fully or partially hydrolyzed. The alcohol residue is preferably selected from aliphatic, saturated, branched or unbranched alcohol residues having 1 to 8 and in particular 1 to 4 carbon atoms. Preferred examples are methanol, ethanol, n-propanol, isopropanol, n-, sec- or tert-butanol or mixtures thereof. The aluminum compound is particularly preferably selected from aluminum oxide or aluminum hydroxide. The at least one silicon compound that must also be present is expediently selected from the group consisting of frits, clay minerals and quartz powder, with the latter being less preferred.

In principle, it has been found in the tests carried out that the use of compounds with a small particle size can promote the formation of the desired mixed crystals in the production of the finishing layer. Accordingly, it is preferred to use the solid components of the glaze composition according to the invention in finely divided form. The metal compound therefore expediently has an average particle diameter D 50 of 1 nm to 30 μm, preferably 50 nm to 500 nm, particularly preferably 50 nm to 200 nm and in particular 50 to 100 nm. In the case of the at least one aluminum compound such as aluminum oxide or aluminum hydroxide this preferably has an average particle diameter D 50 of 1 nm to 30 μm and in particular of 1 to 6 μm. A maximum particle size of 30 μm for the aluminum compound is also preferred because with larger particle diameters the aluminum compound no longer forms eutectics with other components of the glaze composition under the usual firing conditions. However, the formation of a eutectic can lower the melting point of the glaze composition, which in turn affects the viscosity of the melt and thus the ability to form the desired mixed crystals. Above all, the interaction of aluminum compounds such as aluminum oxide with zinc compounds such as zinc oxide promotes mixed crystal formation.

As already mentioned several times, the formation of mixed crystals, in whose crystal structure metal atoms of the at least one antimicrobial metal compound are incorporated, is important in order to achieve a high photocatalytic and antimicrobial activity of the surface finish of a ceramic object produced from the glaze composition according to the invention. Crystal formation can be promoted by suitably adjusting the viscosity of the melt formed during the firing process. A possibility of adjusting the viscosity of the melt, which is known per se, consists in a suitable choice of the concentration of the melt phase formers or fluxes in the glaze composition. It has already been mentioned that some of the glaze components used as antimicrobial metal compounds such as zinc oxide in higher amounts of, for example, more than about 8 and in particular more than 10% by weight itself acts as a flux. A suitable choice of the amount of the at least one antimicrobial metal compound can therefore in some cases mean that no further fluxes have to be added to the glaze composition. In other cases, at least one melt phase former/flow agent is additionally added to the glaze composition. Preferred melt-phase formers/fluxes are, for example, alkali metal and/or alkaline earth metal compounds, since it has been found that they can also improve the photocatalytic effect of surface finishing. This is attributed to the fact that the alkali and alkaline earth metals, of which sodium, potassium, magnesium, calcium and barium are preferred, promote the formation of the mixed crystals and are incorporated into the crystal structure themselves to form active basic silicates, thereby increasing the antimicrobial and photocatalytic properties evoke activity.

Which of these melt phase formers/fluxes are actually used depends, among other things, on whether a glaze composition with an alkaline or an acidic pH is to be produced. As already mentioned, it is preferable, if possible, to use acidic or at least neutral components for acidic glaze compositions, but preferably alkaline or at least neutral components for alkaline glaze compositions. For example, alkali metal and/or alkaline earth metal halides, in particular chlorides, are suitable. In the case of a basic glaze composition, suitable basic melt phase formers or fluxes are preferably selected from alkali metal or alkaline earth metal oxides, hydroxides, carbonates, phosphates, silicates or tungstates. Combinations of melt phase formers/fluxing agents can also be used. The quantity with which the melt phase former/the flux is added to the glaze composition is - as mentioned - selected in such a way that oxidic mixed crystals can form in the melt of the glaze composition at the selected firing temperature within the firing time, the dissolution of the antimicrobial metal compound(s) in a glassy melt, on the other hand, is prevented. Excessively high proportions of melt phase former/flux promote such an undesirable dissolution of the antimicrobial metal compound in the glass matrix. Appropriate amounts of melt phase former/fluxing agent can be readily determined by simple experimentation. They are usually at most 40% by weight, preferably up to 30% by weight and in particular at most 20% by weight or at most 15% by weight, based on the total glaze composition. Glaze compositions that are to be fired at a comparatively low temperature and/or in rapid firing generally require a higher proportion of melt phase formers/fluxes than those that are fired at a higher temperature.

As already mentioned, a frit can also be added to the glaze composition. This can take place as an alternative to or in addition to the addition of the melt phase former. A frit containing the metal of the at least one antimicrobial metal compound is preferably added. The frit then serves not only to adjust the viscosity of the melt, but is also suitable for improving the antimicrobial effect of the surface finish produced. The frit will usually be present in the glaze composition in an amount of up to 65% by weight, in particular at most 60, 50, 40, 30 or 25% by weight. Here, too, the appropriate amount can easily be determined by simple experiments. In all cases, when adding further compounds, care must be taken to ensure that the specified ratio of aluminum to silicon of more than 0.3 is not shifted outside the specified range as a result of the addition.

The invention may also be practiced by modifying glaze compositions known in the art to have the above-described properties of an appropriate aluminum to silicon ratio, a non-neutral pH, and a metal content of at least one antimicrobial metal compound of at most 20 percent by weight, so that they are able to form the desired mixed crystals and thus a high photocatalytic and antimicrobial activity on the ceramic object to be refined during the firing process. Usually this will mean increasing the proportion of aluminum by adding a suitable aluminum compound such as aluminum oxide or aluminum hydroxide. At the same time, a shift in the pH value into the alkaline range can also be achieved in this way. Aluminum compounds such as aluminum oxide in particular also have an influence on the viscosity of the melt of the glaze composition formed during firing and prevent the melt from melting too early or being too liquid, in which crystallization and formation of the desired mixed crystals cannot take place. If necessary, the proportion of the antimicrobial metal compound can be reduced by increasing the other components of the glaze composition, whereby a suitable viscosity of the melt can also be adjusted by means of components such as fluxes in order to promote the formation of the mixed crystals, if this should be necessary.

Depending on the selection of the at least one antimicrobial metal compound, the active properties of the surface finishing layers produced using the glaze composition according to the invention can be adjusted in such a way that the photocatalytic or the antimicrobial properties predominate. The use of calcium compounds enhances the photocatalytic properties, while zinc compounds produce both photocatalytic and antimicrobial properties. In addition to zinc compounds, compounds of the metals lanthanum, molybdenum and tungsten also lead to photocatalytic properties of the surface finishes produced with the glaze compositions according to the invention. In some cases, however, these photocatalytic properties are not quite sufficient, so that, according to the invention, titanium compounds are additionally used in order to impart pronounced photocatalytic properties to the glaze composition and the surface finish produced from it.

The titanium compound can be added to the glaze composition, for example, in the form of titanium dioxide. In some cases, however, this leads to a milky clouding of the finish layer formed. According to the invention it is therefore preferred to add the titanium compound in the form of a titanium alcoholate. The addition usually takes place in an aqueous or alcoholic solvent, so that the titanium alcoholate is optionally present in at least partially hydrolyzed form. In principle, all suitable alcoholic solvents are those which are suitable for dissolving the selected titanium alcoholate. Aliphatic saturated, branched or unbranched alcohols, preferably having 1 to 8 and in particular 1 to 4 carbon atoms, are preferred. Methanol, ethanol, n-propanol, isopropanol, n-, sec- or tert-butanol or mixtures thereof are particularly preferred. The alcoholic residues of the titanium alcoholate are preferably derived from the alcohols already mentioned as solvents. A single titanium alcoholate compound is preferably used in the composition, but mixtures of different titanium alcoholates can also be used in the process according to the invention, particularly if non-hydrolyzed, partially hydrolyzed and/or completely hydrolyzed titanium alcoholates are present side by side. In order to achieve the desired photocatalytic activity, an amount of the titanium compound of up to 5% by weight, based on the total glaze composition, is usually sufficient. Preference is given to 0.01 to 5% by weight, in particular 0.05 to 3% by weight, particularly preferably 0.05 to 2.5% by weight. Calculated for the element titanium, the glaze composition contains preferably 0.01 to 4.5% by weight, expediently 0.02 to 3% by weight, particularly preferably 0.02 to 2.5% by weight and in particular 0.02% up to 2% by weight titanium. As mentioned at the beginning, the quantity is calculated without taking the solvent into account. The titanium contained in the glaze composition is integrated into the crystal structure of the mixed crystals that are formed during the firing process. The extraordinarily good photocatalytic properties of the refinement according to the invention are again attributed to the fact that the titanium, like the other active components, is present in the form of mixed crystals and not in rutile or anatase form and is enriched on the surface of the refinement layer.

In principle, the glaze composition according to the invention can be applied in any manner known in the prior art. The glaze composition is preferably slurried in a suitable solvent before application to the substrate surface to be finished. Aqueous solvents such as water-alcohol mixtures and in particular pure water are preferred. The slurry can be applied to the substrate surface in any suitable manner, for example by steam atomization, pouring, spinning, flow-coating, dipping, rolling or printing or, which is preferred according to the invention, by spraying. By spraying, particularly even applications can be achieved with a very small layer thickness. Because of the high activity of the glaze composition according to the invention, very small layer thicknesses are already sufficient, and these have the advantage that they are practically no longer visible on the substrate surface after baking and accordingly do not change the visual impression of the substrate. The spraying can take place, for example, by means of airless or HVLP spraying. The airless spraying process is carried out, for example, with a membrane pump and a pressure of up to 300 kPa or also with high pressure of up to 30,000 or 40,000 kPa, the slurry being applied to the substrate without the action of an atomizing agent. In the HVLP (High Velocity Low Pressure) process, the atomization pressure is generally in the range of 700 to 1300 kPa and in particular around 1000 kPa. The basic procedure is known to those skilled in the art. Since only a small layer thickness has to be achieved, a single spraying process is usually sufficient. However, if necessary, several spraying processes can be carried out one after the other.

It is usually applied to a ceramic object that has not yet been fired (green compact) or to a pre-fired ceramic object, for example after biscuit firing. Also possible is a surface refinement of an already fully fired ceramic object, which is not preferred from the cost point of view, since when applied to a green body or a prefired ceramic article, the firing of the article and the glaze composition can be done simultaneously, thereby saving energy. In accordance with the method of the present invention, the glaze composition slurry applied to the surface of the ceramic article to be finished is then fired. If desired, the application can be dried beforehand, but this is not usually necessary since the small amounts of the aqueous solvent in the kiln evaporate very quickly at the beginning of the kiln process. The firing process itself takes place in the manner customary in the prior art for glaze compositions and generally at a temperature between 650 and 1350.degree. C., preferably at 800 to 1250.degree. The glaze composition can be fired both in short firing and in long firing, with the duration of firing usually being between 0.5 and 5 hours for short firing and between 5 and 40 hours for long firing.

In principle, the glaze composition according to the invention is suitable for finishing practically any conceivable ceramic object whose surface is to be given improved surface properties, so that it offers protection against aggressive environmental or chemical influences, becomes water, dirt or lime-repellent or easy to clean. The surface can be completely or only partially provided with a surface finish. Surface finishing can be carried out on fired, partially fired or unfired ceramic substrates such as green ceramic bodies, clay or loam, as well as engobed, glazed or unglazed substrates. Suitable ceramic substrates come from the field of sanitary ceramics, building ceramics or laboratory ceramics. Concrete examples are roof tiles, tiles (both for floors and walls), slabs, facade panels, fireclay, chimney pipes, kitchen worktops, kitchen sinks or the like. A particularly preferred ceramic substrate is fine stoneware, for example in the form of wall or floor tiles, which - like the other coated ceramic objects - is given a hard-wearing and, in particular, dirt-repellent surface thanks to the surface finishing layer according to the invention, which is also self-cleaning due to the photocatalytic properties.

The antimicrobial and photocatalytic properties of the surface finishing layer produced using the glaze composition according to the invention are based primarily on the formation of mixed crystals of titanium and/or the metal of the antimicrobial metal compound with silicon and/or aluminum and oxygen. If alkali or alkaline earth metal-containing compounds were also added to the glaze composition as melt phase formers or fluxes, the mixed crystals formed also contain the corresponding alkali or alkaline earth metal elements in the crystal lattice. If the antimicrobial metal compound is a zinc compound, mixed crystals are formed that correspond to the formula 2 ZnO SiO ₂ , with other metal atoms such as the alkali or alkaline earth metals already mentioned and/or titanium possibly being added - depending on the overall composition of the glaze composition the crystal lattice are built in. During the firing process, mixed crystals of different composition are usually formed, distributed over the surface finishing layer. The formation of uniform mixed crystals of the same composition over the entire surface is rather the exception. If the metal of the antimicrobial metal compound is zinc, various zinc-titanium-silicon-aluminium oxide crystals and/or alkali/alkaline earth zinc-titanium-silicon are formed -Alumina crystals. Alternatively or additionally, zinc titanium silicon aluminum hydroxide crystals and/or alkali/alkaline earth zinc titanium silicon aluminum hydroxide crystals are also formed.

It is surprising and advantageous for a pronounced antimicrobial and photocatalytic activity that the mixed crystals generally form with a concentration gradient of the active metal elements such as zinc and titanium, which are essentially concentrated in an area that points to the outer surface of the layer. The concentration of the metal atoms then decreases in the lower-lying areas from a depth of around 1 µm in the finishing layer. The photocatalytically and/or antimicrobially active metal elements are therefore present in the mixed crystals exactly where they are supposed to develop their activity, namely on the outermost surface of the finished ceramic object. The cause of this concentration gradient of the active metal is assumed to be that during the firing process the material on the surface of the ceramic object initially forms a vitreous layer with components of the glaze composition. A dedicated layer boundary between the surface of the ceramic object and the finishing layer can hardly or not at all be discerned in the finished ceramic object. The active mixed crystals then grow out of this vitreous layer formed on the surface of the ceramic object and are enriched with the active metal during the crystallization process. The height at which the mixed crystals grow out of the vitreous layer is usually at most 100 μm, in particular at most 50 μm and predominantly not more than 25 μm, measured perpendicularly to the surface of the ceramic object.

The invention will be explained in more detail below with reference to some exemplary embodiments. The exemplary embodiments relate only to a few preferred exemplary embodiments of the invention, without the invention being restricted to these. Unless expressly stated otherwise, all percentages are percentages by weight.

Example 1 - Calcium Containing Active Glaze Composition

• a) Mit Zink, Titan und Magnesium modifizierte Glasurzusammensetzung

SiO₂ 45,2 % Al₂O₃ 19,0 % CaO 8,1 % Na₂O 3,6 % K₂O 2,7% MgO 9,0 % ZnO 7,4 % TiO₂ 5,0 % Starting from a calcium-containing glaze composition (Protecta) of the following composition: SiO ₂ 55.6% _{Al2O3 _} 23.3% CaO 10.0% Well ₂ O 4.5% _K2O 3.3% MgO 3.3% this was first modified by adding a flux (magnesium oxide) and then by adding an antimicrobial metal compound (zinc oxide) and a titanium compound (titanium isopropoxide), resulting in the following modified glaze composition:

• a) Glaze composition modified with zinc, titanium and magnesium

SiO ₂ 45.2% _{Al2O3 _} 19.0% CaO 8.1% Well ₂ O 3.6% _K2O 2.7% MgO 9.0% ZnO 7.4% TiO ₂ 5.0%

After being slurried in water, the glaze composition a) is applied to the surface of a ceramic object to finish it and, after firing, results in a finish with a basic pH value (pH > 8, measured with an aqueous bromothymol blue solution) and antimicrobial and photocatalytic properties.

Example 2 - Calcium Containing Active Glaze Composition

In this example of another glaze composition containing calcium, the antimicrobial metal compound is added in the form of zinc carbonate: SiO ₂ 47.7% _{Al2O3 _} 20.1% CaO 8.6% Well ₂ O 3.8% _K2O 2.9% ZnCO ₃ 12.4% TiO ₂ 4.5%

The zinc carbonate converts to zinc oxide during firing. Surfaces with antimicrobial and photocatalytic properties are formed.

Example 3 - Barium and calcium containing active glaze composition

Starting from a barium and calcium containing glaze composition, which also contains zinc in the form of a zinc containing frit and which has the following components: SiO ₂ 47.9% _{Al2O3 _} 20.2% BaO 18.1% ZnO 4.3% Well ₂ O 4.2% _K2O 3.2% CaO 2.1% For example, an active glaze composition is prepared by adding titanium isopropoxide and a zinc compound that is not in the form of a frit. Specifically, 6% zinc oxide is added, which leads to an active glaze composition with the following components: SiO ₂ 43.3% _{Al2O3 _} 18.3% BaO 16.4% ZnO 9.6% (of which approx. 3.8% from frit containing zinc) Well ₂ O 3.8% _K2O 2.9% CaO 1.9% TiO ₂ 3.8%

A surface finish made using this glaze composition exhibits high antimicrobial and photocatalytic activity.

Example 4 - Barium and calcium containing active glaze composition

Starting from a barium- and calcium-containing glaze composition, which contains zinc in the form of a zinc-containing frit and, in addition, magnesium oxide and which has the following components: SiO ₂ 45.0% _{Al2O3 _} 19.0% BaO 17.0% ZnO 4.0% Well ₂ O 4.0% _K2O 3.0% CaO 2.0% MgO 6.0% an antimicrobial and photocatalytic glaze composition is prepared by adding a zinc compound which is not in the form of a frit, namely zinc oxide, and titanium isopropoxide. An active glaze composition is obtained with the following components: SiO ₂ 39.8% _{Al2O3 _} 16.8% BaO 15.1% ZnO 8.0% Well ₂ O 3.6% _K2O 2.7% CaO 1.7% MgO 8.8% TiO ₂ 3.5%

The components of the glaze compositions mentioned in the above examples do not necessarily have to be added in the form of the oxides mentioned. Titanium was added in all examples in the form of titanium isopropoxide in isopropanol/water. The amounts used (without solvent) were converted into the corresponding amounts of titanium dioxide in the examples. In addition, one, several or all of the other components mentioned can be added in the form of other metal compounds, which can be converted into the corresponding oxides during the firing process. Examples of suitable metal compounds have already been mentioned in the previous description, in particular carbonates, halides, hydroxides, nitrates, phosphates, sulfates, etc. It is also possible to add the components of the glaze composition at least partially in the form of naturally occurring rocks that have previously been ground into powders. For example, carbonate rocks and silicate-poor and/or aluminum-rich rocks, in particular igneous rocks such as bentonite, calcite, diabase, dolomite, foidsyenite, kaolin, melilite, montmorillonite, mullite, nepheline, olive, plagioclase, pyroxene and syenite are suitable. These are used in amounts such that conversion into the oxides gives the percentages given in the examples.

Example 5 - Application and Firing of the Glaze Compositions

Before application, the glaze compositions are slurried in water. About 0.2 to 5.0, preferably 0.4 to 3.0, in particular 0.5 to 1.8 parts by volume of water are added per part by volume of glaze composition.

• Sprühen: Sprühdruck 100 - 1000 kPa, bevorzugt 400 - 700 kPa Nadel variabel, Durchmesser 0,3 bis 2,5 mm, bevorzugt 0,5 bis 1,5 mm

A raw and unfired green tile body produced in a manner customary in the prior art is dried until its water content is at most 1 to 8%. This is followed, if appropriate, by decoration and the application of the suspended glaze composition according to the invention to the green tile body. The application is done in a single layer. A multi-layer application is possible, but - apart from sanitary glazes - not preferred in order to obtain the thinnest possible layer. The temperature of the green tile compact during application is 15 to 150°C, preferably 40 to 80°C. The slurried glaze composition is applied by spraying according to the following parameters:

• Spraying: Spray pressure 100-1000 kPa, preferably 400-700 kPa Variable needle, diameter 0.3 to 2.5 mm, preferably 0.5 to 1.5 mm

The amount of the composition is 10 to 350 g/m ² , preferably 150 to 250 g/m ² .

• Kurzbrand: Temperatur: 1050 °C bis 1280 °C, bevorzugt 1120 °C bis 1250 °C Zeit (Einfahrt bis Ausfahrt): 0,5 h bis 5 h, bevorzugt 0,5 h bis 2 h
• Langbrand: Temperatur: 1000 °C bis 1300 °C, bevorzugt 1100 °C bis 1250 °C Zeit (Einfahrt bis Ausfahrt): 10 h bis 40 h, bevorzugt 12 h bis 24 h

After application of the glaze composition, the green body is optionally dried again, in which case it is then heated to a temperature in the range from room temperature (20°C) to 300°C, preferably from 35 to 150°C. Drying and heating of the green body is not necessary, especially in cases where the furnace has a drying zone in the heating zone at the beginning of the furnace section. The green body is transported into a kiln in a heated state or alternatively at room temperature to be fired. The burning process takes place in a manner that is customary in the prior art, for example in accordance with one of the following alternatives:

• Short firing: Temperature: 1050° C. to 1280° C., preferably 1120° C. to 1250° C. Time (entrance to exit): 0.5 h to 5 h, preferably 0.5 h to 2 h
• Long firing: Temperature: 1000° C. to 1300° C., preferably 1100° C. to 1250° C. Time (entrance to exit): 10 h to 40 h, preferably 12 h to 24 h

After leaving the oven, the tile is ready and can be packaged, optionally after allowing it to cool to room temperature. In all the examples described, the height at which the mixed crystals protrude over the adjacent surface is less than 25 μm. The finishing layer is with bare Not visible to the eye and does not lead to any visual impairment compared to an unfinished tile.

For some surface finishing layers produced according to the invention, elemental analyzes of the crystalline surface were carried out using energy-dispersive X-ray spectroscopy (EDRS, EDX). The results for glazes A, B and C are summarized in Table 1 below. All glazes have an antimicrobial and pronounced photocatalytic effect. Table 1 A B C O 36.4 38.9 36.6 si 16.3 16.8 15.0 Al 8.3 7.69 9.4 C 8.1 14.6 3.6 N 5.2 4.6 4.0 N / A 2.1 2.4 1.6 K 0.8 0.0 1.4 mg 0.0 0.7 0.6 Approx 2.5 2.8 2.5 ba 11.1 0.0 14.0 Zn 7.7 11.5 8.3 Ti 1.5 0.01 3.0

• 1 eine herkömmliche bariumhaltige Glasur;
• 2 ein weiteres Beispiel einer herkömmlichen bariumhaltigen Glasur;
• 3 eine herkömmliche calciumhaltige Glasur;
• 4 eine herkömmliche Sanitärglasur;
• 5 eine erfindungsgemäße Oberflächenveredelung aus einer calciumhaltigen Glasurzusammensetzung mit Titanisopropoxid und Zinkoxid;
• 6 eine erfindungsgemäße Oberflächenveredelung aus einer bariumhaltigen Glasurzusammensetzung mit Titanisopropoxid und Zinkcarbonat;
• 7 eine Sanitärglasur aus einer erfindungsgemäßen Glasurzusammensetzung mit Titanisopropoxid und Zinkoxid;
• 8 eine erfindungsgemäße Oberflächenveredelung aus einer bariumhaltigen Glasurzusammensetzung mit Titanisopropoxid und Zinkoxid;
• 9 eine weitere erfindungsgemäße Oberflächenveredelung aus einer bariumhaltigen Glasurzusammensetzung mit Titanisopropoxid und Zinkoxid;
• 10 eine weitere erfindungsgemäße Oberflächenveredelung aus einer bariumhaltigen Glasurzusammensetzung mit Titanisopropoxid und Zinkoxid;
• 11 eine weitere erfindungsgemäße Oberflächenveredelung aus einer bariumhaltigen Glasurzusammensetzung mit Titanisopropoxid und Zinkoxid;
• 12 eine erfindungsgemäße Oberflächenveredelung aus einer bariumhaltigen Glasurzusammensetzung mit Titanisopropoxid, Zinkoxid und Strontiumcarbonat sowie
• 13 eine erfindungsgemäße Oberflächenveredelung aus einer bariumhaltigen Glasurzusammensetzung mit Titanisopropoxid, Zinkoxid und Lithiumwolframat.

Electron micrographs were made of the outer surfaces of surface finishing layers produced according to the invention, which as 5 until 13 are attached. 1 until 4 show for comparison electron micrographs of conventional surface finishes. In the recordings, the image section corresponds to 150 µm by 100 µm in the actual surface structure. Show in detail:

• 1 a conventional barium glaze;
• 2 another example of a conventional barium-containing glaze;
• 3 a conventional calcium-containing glaze;
• 4 a conventional sanitary glaze;
• 5 a surface finish according to the invention from a calcium-containing glaze composition with titanium isopropoxide and zinc oxide;
• 6 a surface finish according to the invention from a barium-containing glaze composition with titanium isopropoxide and zinc carbonate;
• 7 a sanitary glaze made from a glaze composition according to the invention with titanium isopropoxide and zinc oxide;
• 8th a surface finish according to the invention of a barium-containing glaze composition with titanium isopropoxide and zinc oxide;
• 9 a further surface finish according to the invention from a barium-containing glaze composition with titanium isopropoxide and zinc oxide;
• 10 a further surface finish according to the invention from a barium-containing glaze composition with titanium isopropoxide and zinc oxide;
• 11 a further surface finish according to the invention from a barium-containing glaze composition with titanium isopropoxide and zinc oxide;
• 12 an inventive surface treatment of a barium-containing glaze composition with titanium isopropoxide, zinc oxide and strontium carbonate and
• 13 a surface finish according to the invention from a barium-containing glaze composition with titanium isopropoxide, zinc oxide and lithium tungstate.

The first thing you notice when looking at the pictures is that the surface structure of conventional surface finishes is very smooth. In contrast to this, the photographs of the surface finishes according to the invention show a strong crystallization which is caused by the mixed crystals formed according to the invention. These mixed crystals are structures of the formula 2ZnO.SiO ₂ and other oxygen-containing crystals which, in addition to silicon, titanium and zinc, contain the other metals contained in the glaze composition incorporated into the crystal lattice. In particular, these are zinc titanium silicon aluminum oxide crystals, alkali/alkaline earth zinc titanium silicon aluminum oxide crystals, zinc titanium silicon aluminum hydroxide crystals and/or alkali/alkaline earth zinc titanium Silicon Aluminum Hydroxide Crystals. In general, mixed crystals of different compositions are distributed side by side over the finished surface. These structured and differentiated surfaces are held responsible for the outstanding antimicrobial and photocatalytic properties of the surface finishing of ceramic objects according to the invention.

In detail shows 5 a surface finish created from calcium-containing glaze compositions and modified by the addition of titanium isopropoxide and zinc oxide. The surface finishing of 5 has the composition of glaze B from Table 1. A surface finish with the composition of glaze A is in 6 to see. It was made from a barium containing glaze composition modified with titanium isopropoxide and zinc carbonate. 7 shows a sanitary glaze whose composition corresponds to glaze C in Table 1. It was made from a glaze composition of the invention modified with titanium isopropoxide and zinc oxide. In the 8th The surface finish shown was created from a barium-containing glaze composition modified with 4.9% by weight zinc oxide and 2.8% by weight titanium isopropoxide. Similar surface finishes are in 9 until 11 pictured. These too were made using barium-containing glaze compositions modified with zinc oxide and titanium isopropoxide. The proportion of zinc oxide in the glaze composition of the surface finish was 9 at 5 , 2 wt%, the 10 at 7 , 4 wt% and the 11 at 6 , 5 wt%. The proportion of titanium isopropoxide in the glaze composition was for surface finishing 9 0.25% by weight, the 10 2.7% by weight and the 11 4.1% by weight. Other surface finishes made using barium-containing glaze compositions are in 12 and 13 shown. According to the surface finish 12 The underlying glaze composition was modified with 5.5% by weight of zinc oxide, 1.8% by weight of titanium isopropoxide and additionally 1.0% by weight of strontium carbonate. The modification of the glaze composition for the surface finishing of the 13 was carried out with 4.9% by weight zinc oxide, 0.8% by weight titanium isopropoxide and 1.5% by weight lithium tungstate.

All surface finishes have photocatalytic properties in addition to antimicrobial properties. In all cases, those in the 5 until 13 recognizable crystal structures have grown out of a vitreous layer which is formed during the firing process by the reaction of the glaze composition with components on the surface of the ceramic object. The height at which the mixed crystals that form grow out of the vitreous layer and protrude over the vitreous layer in the finished surface finish is in all cases less than 100 µm and is usually less than 50 µm and in particular at most 25 µm, measured in the vertical Direction to the surface of the ceramic object.

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• EP 3053902 A1 [0002]

Claims (15)

Glaze composition with an antimicrobial and photocatalytic effect for the surface finishing of a ceramic object, which comprises at least one silicon compound, at least one aluminum compound, at least one titanium compound and at least one antimicrobial metal compound, the metal of the at least one antimicrobial metal compound being selected from the group consisting of zinc, tin, copper, calcium, strontium, lanthanum, cerium, iron, nickel, vanadium, molybdenum and tungsten, characterized in that the weight ratio of aluminum to silicon in the glaze composition is greater than 0.3, that the proportion of the metal of the at least one antimicrobial metal compound in the glaze composition is at most 20% by weight and that an aqueous slurry of the glaze composition has a pH value other than neutral. Glaze composition according to any one of the preceding claims, characterized in that the weight ratio of aluminum to silicon in the glaze composition is in the range from 0.3 to 5 and in particular from 0.4 to 1.2. glaze composition claim 1 or 2 , characterized in that the at least one titanium compound is a titanium alcoholate which can optionally be at least partially hydrolyzed, the alcohol residue preferably being selected from aliphatic saturated, branched or unbranched alcohol residues having 1 to 8 and in particular 1 to 4 carbon atoms, in particular methanol , ethanol, n-propanol, isopropanol, n-, sec- or tert-butanol or mixtures thereof. glaze composition claim 3 , characterized in that the at least one titanium compound is contained in the glaze composition in an amount of up to 5% by weight. Glaze composition according to any one of the preceding claims, characterized in that the pH of the slurry is in the alkaline range and in particular is greater than 7.2, preferably greater than 7.5 and most preferably greater than 8. glaze composition claim 5 , characterized in that the metal of the antimicrobial metal compound is selected from zinc, strontium, iron, copper and nickel and is preferably zinc. glaze composition claim 5 or 6 , characterized in that the at least one antimicrobial metal compound is selected from the group consisting of oxides, hydroxides, carbonates, nitrates, sulfates, chlorides or mixtures thereof, and preferably zinc oxide, zinc hydroxide or zinc carbonate, where optionally the zinc compound is zinc oxide and with a maximum of 10% by weight, preferably 3 to 7% by weight, is contained in the glaze composition. Glaze composition according to one of Claims 5 until 7 , characterized in that it further contains at least one of the following components: a) a basic flux, preferably an alkali metal or alkaline earth metal salt, in particular an alkali metal or alkaline earth metal oxide, hydroxide, carbonate, phosphate, silicate or tungstate, wherein the basic flux is optionally contained in the glaze composition in an amount of at most 40% by weight, b) a frit, preferably a frit, which contains the metal of the at least one antibacterial metal compound, the frit optionally being present in an amount of up to 65 % by weight is contained in the glaze composition. Glaze composition according to one of Claims 1 until 4 , characterized in that the pH of the slurry is in the acidic range and is in particular less than 6.8, preferably less than 6 and particularly preferably less than 5. glaze composition claim 9 , characterized in that the metal is tin and the antimicrobial metal compound is selected in particular from tin oxide, tin hydroxide and tin chloride. Glaze composition according to one of the preceding claims, characterized in that the at least one aluminum compound is selected from the group consisting of aluminum oxide, aluminum hydroxide, aluminum nitrate and aluminum alcoholates, which can optionally be at least partially hydrolyzed, the alcohol residue preferably being selected from aliphatic saturated branched or unbranched alcohol radicals having 1 to 8 and in particular 1 to 4 carbon atoms, in particular methanol, ethanol, n-propanol, isopropanol, n-, sec- or tert-butanol or mixtures thereof, and/or that the at least one silicon compound is selected from the group consisting of frits, clay minerals and quartz flour. glaze composition claim 11 , characterized in that the aluminum compound is aluminum oxide or aluminum hydroxide and has an average particle diameter D 50 of 1 nm to 30 µm, in particular 1 to 6 µm, and/or the metal compound has an average particle diameter D 50 of 1 nm to 30 µm, preferably 50 nm to 500 nm, particularly preferably 50 nm to 200 nm, and in particular from 50 to 100 nm. Process for finishing the surface of a ceramic object with an antimicrobial and/or photocatalytic surface finishing, characterized in that an aqueous suspension of the glaze composition according to one of Claims 1 until 12 applied to a surface of the ceramic article and then fired. Ceramic object obtainable by the process according to Claim 13 , characterized in that it is sanitary ceramics, building ceramics, in particular a roof tile, a tile, a slab, a facade panel, fireclay, a chimney pipe, a kitchen worktop or kitchen sink, or laboratory ceramics. Ceramic object according to Claim 14 , characterized in that the coating has at least one of the following properties: - it contains mixed crystals of titanium and / or the metal of the antimicrobial metal compound with silicon and / or aluminum and oxygen, optionally • the mixed crystals, if the metal is zinc, such of 2 ZnO.SiO ₂ , optionally with alkali or alkaline earth metals and/or titanium built into the crystal lattice, • the mixed crystals, if the metal is zinc, in particular zinc-titanium-silicon-aluminium oxide crystals and/or alkali/alkaline earth - are zinc-titanium-silicon-aluminium oxide crystals, • the mixed crystals when the metal is zinc, in particular zinc-titanium-silicon-aluminium hydroxide crystals and/or alkali/alkaline earth zinc-titanium-silicon-aluminium hydroxide crystals, • the mixed crystals have a height, measured perpendicular to the surface of the ceramic object, of no more than 100 µm, preferably no more than 50 µm and in particular no more than 25 µm, have, - in the case of a glaze composition according to any one of Claims 5 until 8th , 11 and 12 the surface of the coating has a basic pH or - in the case of a glaze composition according to one of claims 9 until 12 the surface of the coating has an acidic pH.

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