The early literature provides many methods for preparing the pigment, most of which seem relatively cumbersome. (See Appendix D, below.) Here is a rapid and straightforward synthesis that produces enough pigment to provide 0.05% Au in a 100-gram test batch of glaze. Doubling the amount of gold chloride provides 0.1% Au, and produces deeper color in the fired glaze.
“Step 0”: Before you start, it is helpful to dissolve 1 gram of AuCl3 in 12 grams of distilled water. 1 gram of the resulting solution contains approximately 50 milligrams of gold, which is required in Step 2 of the preparation.
Step 1: Suspend 5 grams of Al(OH)3 (Aluminum Hydroxide, often inaccurately referred to as “Alumina Hydrate”) in approximately 12 grams of distilled water (the amount is not crucial) at room temperature, with stirring. Other substrates may be used in place of Al(OH)3, but see Note 2, below.
Step 2: Continue stirring the resulting suspension, and add 50 mg of gold in the form of an aqueous solution of the chloride. If you are using a solution prepared as in Step 0 above, you will require 1 gram.
Step 3: Add, dropwise, an aqueous solution of SnCl2 (Stannous Chloride), stirring with each addition, until no further change in color is observed. It is a good idea to add a slight excess, as the change in color becomes progressively more difficult to observe as the color darkens. Solid SnCl2 can be used instead of a solution, but the solution is more convenient.
The resulting suspension can be added directly to 95 grams of other materials to produce a 100-gram glaze batch. (See Appendix C for an example recipe.)
1. This preparation produces mauve stains. I have not investigated further, but the colloidal gold colorant is known to be capable of producing a surprisingly wide range of colors, depending on conditions. The color depends on the particle size, and possibly other parameters.
2. The pigment can be deposited on a wide variety of substrates. Aluminum hydroxide is convenient, as is alumina, and the resulting color is satisfactory with either. Kaolin is used in Hermann Seger’s preparation, which is given in Appendix D, and should work well in this preparation. Pigment deposited on Nb2O5, however, did not produce significant color in the fired glaze when I tested it. Pigment deposited in an aqueous solution of ZrOCl2 has remained in suspension for several years; I have not yet attempted to use it in a glaze.
I have not tried working with larger quantities, but I
have no reason to expect any problems with scaling.
The following glaze produces consistent mauve color with 0.05% or 0.1% Au (prepared as in Appendix A and added as a stain), when fired to Orton cone 10 or 11 in a mildly reducing atmosphere. When fired to cone 9 in air it produces an extremely pale pinkish color.
Note: This recipe is merely a representative testing formulation, and is not intended to be prescriptive.
Kaolin 20g (Any good primary kaolin; I use Super Standard Porcelain) Petalite 30g Wollastonite 25g MgCO3 5g Fusion Frit F-69 15g (or Ferro Frit 3249) Mauve Stain 5g (plus small amounts of Au and Sn from preparation) Total: 100g (ignoring the Au and Sn)Seger numbers for this glaze, as calculated by Insight:
Al2O3: 0.465 SiO2: 2.357 B2O3: 0.175 K2O: 0.010 Na2O: trace Li2O: 0.132 CaO: 0.582 MgO: 0.276 Fe2O3: 0.002 TiO2: 0.003
“Very beautiful purple and rose tints are obtained by introducing gold as a coloring metallic oxide [sic]. A strong purple-red is prepared in the following manner: Of pure white kaolin, best English china clay, 90 parts [sic] are worked up in water by boiling and sieving; then 10 grams of gold are dissolved in a mixture of nitric and hydrochloric acids, the solution being freed from excess of acid as much as possible by evaporation on the water-bath. The gold solution is then mixed with the clay slip, sodium carbonate is added till the liquid is distinctly alkaline, and after an addition of about 20 grams of grape-sugar the mixture is boiled strongly for half an hour, water being added to maintain the same volume. Thus a dark purple colored mass is obtained, which must be washed and then dried and ignited at a heat not exceeding silver-melting heat. It will in this way assume a somewhat lighter color. A second washing after the grinding of the color is desirable since the first washing is never as complete as is necessary.
“A beautiful rose color is thus obtained from
98 parts [sic] of white porcelain clay
2 grams of gold”
1. The melting point of silver is just under 962° C.
2. This method, while somewhat annoying to follow, is not actually unreasonable. Gold chloride can be purchased, which eliminates the need for strong acids; potassium carbonate (pearl ash) can be used interchangeably with sodium carbonate (washing soda); fructose can be substituted for grape-sugar; it is convenient to heat the suspension in a microwave oven; and the addition of even a very small amount of SnCl2 prior to the addition of the sugar decreases the required boiling time from 30 minutes to roughly 60 seconds (at least in the microwave oven), the exact time depending on the substrate being used and possibly other factors. The one real problem is that the amounts of clay are given as parts, whereas the amounts of other materials are given as grams. (This may be an artifact of the translation.)
3. For testing purposes, it is not necessary to calcine the pigment. Just mix it into the test glaze.
Collected Writings, Vol. 2 (translated into English, 1902), page 664
The Chemical Publishing Co., Easton PA
Last modified: Sat Oct 27 00:01:19 EDT 2012