Overglaze enamels, as their name implies, are made to be applied over other enamels. They act much like paints or inks. While my experience is in enameling steel, I believe this information should be useful to enamelists who work on other metals as well.
After a piece of steel has been enameled with ground coat and finish coat, one of the options for adding decoration is to use overglaze enamels. While premixed overglazes are available, I like to mix my own. It allows me to fine tune them for my own needs. It also allows me to make the amount I need when I need it. And it is easy to do.
In the enameling industry, what we are about to make is called paste. However, I like to call it paint or ink depending on how it is being used. Call it paste, paint, or ink, it is the same material, and I will use all three terms here.
To make an overglaze you need three ingredients: a frit, a colorant, and a medium. The frit is the glass base. An important distinction for overglaze frits is they are milled very fine like talcum powder so they will not seem gritty when used as a paint. Both finely ground white and clear are available. Colorants*, as their name implies, are commercially prepared powders that add color to the frit. A medium is a liquid that turns the powdered frit and colorants into a paint or ink.
Making a Paste
Making a paste is easy. In addition to the ingredients, you will need a porcelain enamel panel or sheet of glass on which to mix the materials, a palette knife and some rags or paper towels. Also, don’t forget to use a dust mask. When ready, place the proper amounts of frit and colorant on the mixing panel. (See Figure 1) Mix in small amounts of medium with the palette knife until the ink is the consistency you want, and the paint is ready to use. (See figure 2) I like to mix overglazes thick like a paste. This reduces settling during storage, and they can be thinned at the time of use.
Sometimes mixing the paste with a palette knife is not enough, and specks of colorant** will show in the ink. This nearly always is true of screen printing inks. To remedy this, use a small silk screen or other screen with a very fine mesh. Push the ink through the screen with a palette knife or a squeegee, and the specks will go away. The easiest way to do this is to turn a silk screen upside down over your mixing plate. (See figure 3)
You can measure materials by volume, but measuring by weight is much more accurate. To mix small amounts, a scale that measures to an accuracy of .01 grams is handy. Most will also weigh in grains. This is useful for very small amounts like a teaspoon full. When adding colorants to a frit, designate the amount of frit as 100% and always base the amount of colorant(s) on that. For example, if you have 100 grams of frit and want to add 9% of colorant, find 9% of 100 grams, and add 9 grams of colorant. The total mixture would now weigh 109 grams. If you decide to add another colorant, disregard the nine grams of color you added and once again base your calculation on the weight of the frit. Thus, to add, 3% of another color, find 3% of the 100 grams of frit and add 3 grams of colorant to the mixture. You now have 100% frit plus 12% colorant for a total of 112 grams of material. The reason for this convention is so the materials that make up the frit will always add up to 100%. This makes life easier for any one who might want to tinker with the makeup of the frit.
The amount of colorant you add to the frit is up to you. If you add very little colorant, the enamel will tend to be transparent. If you add too much, the overglaze will not melt at the proper temperature. Since colorants are more expensive than frits, add as little color as you need to get the color you want. As a starting point, consider using 5% or 10% colorant. Colorants like any other material in an enamel will affect it in more than one way. Adding too much colorant may affect the firing temperature, durability or scratch resistance.
More About Materials The Frit
I encourage you to experiment with different materials and share your experience with other enamelists. These are the materials with which I am most familiar.
I use an unleaded frit called Ferro frit 9636. This frit is fairly transparent, and when very small amounts of colorant, like .1 percent, are added to it, makes translucent colors. If more colorant is added, opaque, dark colors will result. You can make a white enamel by adding titanium to this frit in the form of either anatase or ilmenite. Ilmenite tends to have a more yellow cast than anatase. Using titanium with colorants will yield light colors. I fire it between 1300 and 1440 degrees. I fire lower for screen printed images with nice sharp edges, and I fire higher for hand painting which tends to have a heavier deposit of material.
The medium you use depends on how you want the enamel to work. Ferro Corporation 175 (Thompson A-8) oil has a pine oil base. It can be thinned with alcohol. It makes good screen printing ink or painting medium. If you are careful, you can paint over it when it dries without having to fire it. A drawback is an odor that some people find objectionable. You can clean brushes with some detergents. Silk screens can be cleaned with a detergent and the help of a power washer.
Ferro Corporation 1368 (Thompson A-14) medium has a glycol base. It has little or no odor and cleans up easily with water. This medium is very versatile. It works as a screen printing ink, cleaning up easily with plain water. When thinned with water or alcohol, it is a good spray medium, again with easy clean up. It is a good hand painting medium, and can be applied heavily or thinned to create water color effects. It dries so slowly it usually must be heated to dry. It cannot be painted over when dry without firing on the first coat. It is worth noting that paints made with 175 oil and 1368 medium can be mixed together.
There are many media available. Some people like sewing machine oil as a medium and even plain water will work for airbrushing. Ask your supplier about others. By the way, take media names with a grain of salt. Suppliers sometimes buy a material and resell it under a different name.
There are colorants made specifically for porcelain enamel. Unfortunately, most of them are difficult to find in small quantities. The companies that make them sell to large plants, so their minimum orders are too large for the average enamelist. Fortunately, you can extend your palette by mixing colors together. Blue and yellow makes green, and so on. Also, while not specifically made for porcelain enamel, stains from the Mason Company work well with a little experimentation. A large variety of these stains are available in small quantities from stores that sell supplies to potters.
Materials needed to mix your own colors in practical amounts are available from Thompson Enamel, 1-859 291-3800, and Clay Art Center, 1-800-952-8030, among others. Scales for weighing small amounts of materials are available on the Internet, especially Ebay. Thompson’s Painting Enamel Kit contains all materials mentioned in this article.
I hope you have found this useful. Mixing your own overglazes really is easy. Give it a try.
*Colorants. Those used for enamel are often called oxides. Those used for pottery are always called stains. Stains quite frequently contain clay as an extender. This does not harm the potter, but usually causes the enameler a problem.
**Specks of Colorant. This is a sign of insufficient mixing of the glass and colorants prior to adding the medium. Commercial manufacturers of screening paste normally ball mill the glass and colorants prior to adding the medium. Small amounts can be mixed without a ball mill using the method taught in GOM Vol. 23, No.4, page 85. Those who do not have this issue can obtain a free copy by contacting GOM, (859) 291-3800, or email@example.com.