Abstract:
The present invention relates to a method for coloring ceramic surfaces, wherein the surfaces are treated with an aqueous solution of alkali metal or ammonium dithiosulphatoaurate(I) with a gold concentration of 0.1-10% by weight, the water is evaporated and the dithiosulphatoaurate(I) is decomposed at temperatures of 300-1400° C.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The subject-matter of the present invention relates to a new method for coloring ceramic surfaces by using the aqueous solution from a gold compound. 
     2. Description of the Related Art 
     The production of pink color shades has proven to be relatively problematic for the ceramic industry. It is standard practice to produce pigment colors by sintering together the most varied metal oxides and applying these to the ceramic surface with boric acid as fluxing means. 
     The reference DD 224026, for example, describes a &#34;pink coloring element&#34; with the approximate composition: 2 CaCO 3  ×SnO 2  ×2SiO 2  ×H 3  BO 3  and containing 0.1-1.5% Cr 2  O 3  as color-causing component. 
     Such pigment powders have the disadvantage that they must either be added to the total ceramic mass, making it impossible to add a pattern, or that they are applied only as a thin layer to the ceramic surface and thus change the ceramic surface, so that a subsequent treatment of the surface is no longer possible. 
     It is furthermore known that colloidal gold, which is stabilized through tin oxide, results in dark ruby-red to lilac-red color shades (Cassius gold purple). The admixture of this gold purple to glasses and glazes consequently results in correspondingly ruby-red to lilac-colored glasses (gold ruby glass) and glazes (S. Stephanov et al. Ceramic Glazes, Wiesbaden/Berlin 1988, page 127). 
     It is furthermore known that when gold sulphoresinates, suspended in an organic solvent, are applied to a ceramic surface and are fired at 500-800° C., this results in a thin coating of metallic gold with a typical gold luster (compare DE-C1-41 22 131). 
     According to the DE-C1 43 20 072, diluted, aqueous solutions of gold salts, particularly gold chlorides such as gold(III)-chloride or tetrachloro gold acid, which can be obtained commercially as monohydrate and trihydrate, can be used to color in the surfaces of ceramic bodies, such as tiles or porcelain. A pink to blue coloration is obtained in a very simple way by calcining the surface of the ceramic body, colored in this way. 
     The disadvantage of this method is that the gold salt solutions used are not stable under normal storage and processing conditions and react in particular with metallic surfaces in the devices used and with reducing impurities or additives by forming amorphous gold, which is precipitated out as &#34;slurry.&#34; 
     The object of the invention therefore was to find a new method for coloring ceramic surfaces, particularly coloring with pink color shades, which method allows the introduction of color at a later date to the surface layer of the ceramic material and the coloring of this surface layer to a depth of about 0.5-2 mm, so that it is possible to apply a pattern as well as use a subsequent treatment on the surface. A stable, non-toxic gold compound is to be used for the method. 
    
    
     SUMMARY OF THE INVENTION 
     The solution is with the features specified in the main claim, as well as the features specified in the dependent claims. 
     In accordance with the invention, the method representing the solution is characterized in that the surfaces are treated with an aqueous alkali metal or ammonium dithiosulphatoaurate(I) solution with a gold concentration of 0.1-10% by weight, that the water is evaporated and the dithiosulphatoaurate(I) is decomposed at temperatures of 300-1400° C. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A dithiosulphatoaurate(I) solution with a gold concentration of 0.5-5.0% by weight is preferred for the method. 
     The method has proven particularly successful if the decomposition of the alkali metal or ammonium dithiosulphatoaurate(I)--in the following mostly referred to as thiosulphatoaurate--takes place at temperatures of 800-1200° C., especially at 1140° C. 
     With the method according to the invention, the aqueous solution of the thiosulphatoaurate can be applied in the standard way through spraying, submerging, painting, printing, etc. to the ceramic bodies to be colored. 
     Following the drying, the ceramic bodies are fired in a suitable kiln at a defined temperature at 300-1400° C., preferably at approximately 800-1200° C., and especially for producing a pink coloration at 1140° C., wherein the firing period can last from about one half hour to five hours, but lasts preferably two hours. The firing pyrolizes the thiosulphatoaurate, and finely distributed elementary gold is formed. The size and distribution of the gold particles are critical for the color impression. This is controlled through the firing temperature and length of time for firing. At temperatures of approximately 300-400° C., the resulting color shade is blue and at temperatures of 400-1000° C. it changes over to lilac, only to result in a pure pink shade at temperatures above 1000° C. The gold concentration of the thiosulphatoaurate solution is also important, since the color shade becomes too pale below approximately 0.1% by weight gold and above 10% by weight gold, the color impression after the firing for metallic, amorphously distributed gold changes to a typical brown shade. A range of 0.3-5% by weight gold in the thiosulphatoaurate solution is therefore preferred. 
     Surprisingly, the alkali metal and ammonium thiosulphatoaurate(I) complexes can be pyrolized under the same conditions as gold(III) chloride. They are resistant to reducing agents such as metallic iron, so that it is possible to use equipment made from iron or iron alloys, such as frames, strainers, jets. 
     The color impression after the firing can be reproduced objectively with the aid of a La*b* system. In this system, L stands for the brightness, whereas a* and b* indicate the color shade as well as the saturation. 
     In this case, a* designates the position on a red-green-axis and b* the position on a yellow-blue-axis. 
     Exemplary Embodiments for the Method According to the Invention Solutions Used: 
     Solution I 
     The solution I is a preparation containing water and 
     6.0 g/l gold as sodium dithiosulphatoaurate(I) 
     40.0 g/l sodium thiosulphate 
     8.0 g/l disodium hydrogen phosphate 
     8.0 g/l sodium dihydrogen phosphate 
     14.0 g/l potassium peroxo monosulphate and 
     8.5 g/l ethylene diamine tetra-acetic acid, tetrasodium salt 
     The solution has a gold concentration of 0.6% by weight and a pH value of 6.0. 
     Solution II: 
     The solution II is a preparation of water and 
     6.0 g/l gold as sodium dithiosulphatoaurate(I) and 
     40.0 g/l sodium sulphite 
     The solution II has a gold concentration of 0.6% by weight and a pH value of 9.8. 
     Solution III: 
     The solution III is a preparation of water and 
     6.0 g/l gold as sodium dithiosulphatoaurate(I) 
     40.0 g/l sodium sulphite and ammonium hydroxide up to a pH value of 11.4. 
     The gold concentration for solution III is 0.6% by weight 
     Tiles used 
     Villeroy &amp; Boch, dark-firing, 54×54 mm (type 1) 
     Villeroy &amp; Boch, light-firing, 45×45 mm 
     Villeroy &amp; Boch, light-firing, 54×54 mm (type 2) 
     The following table 1 reflects the results of color measurements on tiles, which were sprayed with two thiosulphatoaurate solutions with a different composition, the solutions I and II, and were fired at three different final temperatures for 60 minutes (in this case, 0.06 g of the thiosulphatoaurate solution applied to 1 cm 2  of tile surface) With the final temperatures of 800° C. and 1140° C., a prefiring occurred respectively at 600° C. for 760 minutes to reduce the thermal stresses; at 400° C. the firing lasted for a total of 300 minutes. It turned out that the color impression changes from the low temperatures (blue) to the average temperatures (lilac) and to the high temperatures (pink). 
     The fired tiles are measured with a &#34;Minolta&#34; Chroma Meter CR-200 (absolute measurements, La*b* color system). 
     
         ______________________________________Experiment 1  Experiment 2 Experiment 3______________________________________T1 = 400° C.         T1 = 600° C.                      T1 = 600° C.T2 = 400° C.         T2 = 800° C.                      T2 = 1140° C.time 1 = 240 min         time 1 = 360 min                      time 1 = 360 mintime 2 = 60 min         time 2 = 60 min                      time 2 = 60 min______________________________________ 
    
     
                       TABLE 1______________________________________results of the color measurementslarge tiles         small tilesL          a*       b*      L      a*   b*______________________________________blank test   71.26  +4.32    +10.43                         79.83  +1.74                                     +8.20400° C.solution I   63.66  +3.07    +4.42 71.12  +2.34                                     +1.09400° C.solution II   61.90  +2.65    +3.36 70.99  +2.25                                     +0.84400° C.blank test   77.91  +8.17    14.26 86.72  +4.46                                     +10.94800° C.solution I   68.89  +9.48    +5.68 75.12  +6.13                                     +0.41800° C.solution II   69.81  +7.95    +5.37 72.71  +5.58                                     -1.60800° C.blank test   59.95  +0.38    +12.50                         79.16  +2.27                                     +15.821140° C.solution I   58.19  +2.58    +10.76                         71.79  +6.47                                     +12.001140° C.solution II   57.76  +3.04    +11.15                         72.19  +6.81                                     +12.281140° C.______________________________________ 
    
     Additional color measurements were carried out on large tiles type 2 (firing temperature 1140° C.). Color differences in the blank tests were detected during this, which also effected the color shade of the colored tile. The tiles of the type 2 show a medium light basic color. 
     
                       TABLE 2______________________________________results of the color measurements on type 2 tiles   L          a*     b*______________________________________blank test     65.28        +0.68  +14.26solution 1     60.80        +4.78  +11.55solution 2     60.12        +5.38  +11.26______________________________________ 
    
     Surprisingly, the nature of the ceramic body appears to influence the coloration only slightly, since tiles and porcelain shards are colored in the same way under the same conditions for application and firing. As shown in table 2, a possible basic coloration of the shard is added to the color shade. 
     The colored ceramic layer has a thickness of approximately 0.5-2 mm, in particular about 1 mm, so that it is possible to sand off rough spots on the shards (approximately 0.3 to 0.5 mm) and polish the surface, without affecting the color impression. In particular with hard-fired tiles, products can be manufactured in this way, which are step-resistant and polished on the surface. Given the corresponding printing techniques, it is possible, for example, to produce floor tiles with a marbled effect, wherein the produced floor tiles are superior to real marble because of their strength and can be produced more economically. 
     Thiosulphatoaurate solutions, which contain additional stabilizers--particularly sulphites or thiosulphates--have proven to be particularly usable. 
     Sodium sulphite and a mol ratio of gold to sulphite of (0.1-3) to 10, preferably (0.5-2) to 10, and especially 1 to 10 in the solution have proven successful. 
     Determination of effective stabilizer concentration 
     An aqueous solution containing 6 g/l gold in the form of sodium dithiosulphatoaurate(I), Na 3  [Au(S 2  O 3 ) 2  ] is produced. Following this, varied amounts of sodium sulphite are added to aliquot shares of the solution, and 5 ml each of the resulting solution are poured into a test tube. An iron nail is placed into each test tube. After 12 hours, a check is made to determine from which solutions gold has been precipitated out and what type of precipitation has occurred. The concentration of gold and sulphite in the solutions, the mol ratio of gold to sulphite and the observed gold deposits are combined in a table 3. 
     
                       TABLE 3______________________________________                          gold precipitatedGold     sulphite              out in theconcentration    concentration               mol ratio  presence of an[g/l][mol/l] [g/l]  [mol/l]                     gold:sulphite                              iron nail______________________________________6    0.03    0      0              voluminous, brown                              deposit6    0.03    10     0.08  3.8:10   thick gilding with                              little brown deposit6    0.03    20     0.16  1.9:10   gilding with very                              little brown deposit6    0.03    30     0.24  1.3:10   thin gilding6    0.03    40     0.32  0.9:10   no precipitation______________________________________