Q. I was curious about the formulation of a paintable gold enamel for blown glass pieces

I consulted Bill Gudenrath (CMoG Studio) who replied: 

"The best think I've seen on what is in the commercially available 'liquid gold' products is "Gold in the Pottery Industry" by L. B. Hunt in Gold Bulletin, v. 12, no. 3, 1979, pp. 116-127. 

I didn't find a copy of this article in our library. You should be able to obtain one via Interlibrary Loan at your local library. 

You may find additional information in these sources: Try contacting the suppliers listed to find information about a gold enamels and firing temperatures for blown glass pieces (Paradise Paints is closed). 

Glass on Metal Magazine provides extensive online information about enamels: https://www.glass-on-metal.com/links/web-resources.htm 
They also have a message board where you could ask for formulations: 
https://www.glass-on-metal.com/messageboardpopup.htm 

"Proper use of Liquid Metals" by Martin Hanson provides technical information about firing on enamels (or glass) 
https://www.glass-on-metal.com/pastart/liquid_metal-hanson.h
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This article, found in their back issues, describes the preparation of enamels: 
https://www.glass-on-metal.com/pastart/enamel_prep.htm 

Information in this article about lusters may be useful: https://www.glass-on-metal.com/pastart/lusters-mcdaniel.htm 

They also list enamel suppliers who may provide information about the process of painting with gold enamel: 
https://www.glass-on-metal.com/suppliers/enamelsuppliers.htm 

There is an "ask the experts" section as well: 
https://www.glass-on-metal.com/ask_the_experts/questions.htm 

 
The Society of Glass and Ceramic Decorators (SGCD) is another wonderful source of information: https://www.sgcd.org/ 
To contact them: 
Society of Glass and Ceramic Decorators 
47 N. Fourth Street / PO Box 2489, 
Zanesville, OH 43702 
740-588-9882 
Fax: 740-588-0245 

They list the following article: Butterfield, "Characterization Techniques for Liquid Gold Product Development." in their Technotebook publication 
E-mail: sgcd@sgcd.org / Contact: Myra Smitley 

Another resource is the Society of Gilders: 
https://www.societyofgilders.org/?action=index 
One of their members may be able to help you. Also, their "links" tab provides addresses for gold products - check the company websites to see if they offer technical information. 

Another article by Hunt, looking at the historical origins of gold enamel: 

Hunt, L. B. “The True Story of Purple of Cassius: The Birth of Gold-Based Glass and Enamel Colours.” Gold Bulletin, v. 9, no. 4, 1976, pp. 134-139, ill. 
(We have a photocopy): Glass Making -- Batches, Recipes, Formulae, Compositions -- Ruby Glass