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First the short version: The more stain you add, the darker the color will be.
GLAZE: Look at the numbers below each color on the Mason Stain page. The numbers 5, 6, 7 or 8 tell whether the stain likes zinc or not. Some glazes have zinc and other will say zinc-free. So match the stain to the glaze you are using.
CLAY AND SLIP: You can also color with Mason Stains.
FRIT: Instead of using a glaze, you can add stains to water and FRIT. FRIT is a powdered combination of ingredients that basically makes up a very simple glaze. It allow a little "melt" to occur, which will bind the mason stains to the clay when fired. FRIT 3134 is commonly used at low temperatures and up to Cone 6. FRIT 3195 is common used for higher temperatures up to Cone 10. http://www.bigceramicstore.com/Supplies/chemicals-frits/ChemicalIndex.htm (to be linked)
Now the long version:
Stains are fired blends of metal and ceramic oxides that have been reground into a fine powder. Stains containing otherwise toxic oxides can be employed without significant dangers. This is the first aspect of something that stains have that coloring oxides don't: stability. A second aspect of stability is that stains produce much more consistent and repeatable color than using raw oxide colors.
Stains are most popular at lower temperatures where colors tend to be brighter. However most stains can be used right up to high fire. Premixed low fire glazes are typically made by blending stains and commercial frits and other than following firing instructions, users of these products give little thought to the technical challenges that were overcome to produce them. This is a third key advantage of stains: the ability to target a specific color. Many ceramic color shades (i.e. reds) are difficult to achieve and beyond the abilities of end users.
While many stains are 'standard' and their composition is well known across the industry, others are proprietary. Stain companies don't release the exact makeup of stains but they do tell us the 'system'. For example, a green stain might contain chrome, cobalt and silica and be labeled 'CrCoSi'. Although silica is not a colorant itself, it is included to create a stable silicate crystal structure with the other two.
Stain companies often supply multiple products to produce a given color using different oxide systems (i.e. chrome-tin pink, manganese-alumina pink). Various factors like the base glaze chemistry, color shade, temperature, and end use determine the system you should choose. Knowing how to calculate the oxide makeup of a glaze or dealing with a stains.
Thus stains do not come with a 'unconditional color guarantee'. The shade produced depends on many of factors including the host glaze chemistry, on/over/underglaze use, glaze thickness, amount of opacifier, firing temperature, etc. Achieving colors with stains is certainly easier, but it is not a 'no-brainer'. Certain systems are quite flexible and produce color in many kinds of glazes (ie. cobalt silicate). Other systems either require that certain oxides be present in the host glaze in minimum amounts or others not be present at all. The symbiosis of host glaze chemistry and stain, for example, can be demonstrated with chrome-tin stains. They will not develop color if zinc is present or if there is inadequate calcia. It is common to hear people say that their pink stain 'burned out', but generally the stain is being used in an incompatible glaze base. Another interesting demonstration of these factors is the color chart of a typical stain company. These charts show the stain used in one or more fritted base glazes that are selected to be compatible with as many of the colors as possible. Certain samples will also have added opacifier and zinc, for example.
Admittedly, stains can produce homogeneous color which can be less interesting than the variegated and speckled color effects that can be achieved with raw metal oxides like iron, cobalt, copper, etc. But for manufacturing, it cannot be ignored that stains are far more consistent and reliable to use.
Potters love to paint stains over and under glazes to decorate ware. Majolica ware is a good example. However, keep in mind that stains vary in their willingness to allow the underglaze to envelope the color particles. Some tend to crystallize the surface if used overglaze. Likewise, if stains are used underglaze they vary in their willingness to allow the overglaze to penetrate through to form and interface with the body. Stains don't suspend well in water to create a paintable material either so it is necessary to mix them with a compatible and paintable glaze medium for doing brushwork.
Stains exist either in the context of the huge industrial ceramic industry or in the hobby, pottery, and ceramics markets. Large industries either have in-house technical people or contract consultants. Small users do not have this luxury. They should know that certain stain companies, (i.e. Mason) have developed excellent reputations for dealing with smaller volumes and providing support.
A fired glaze can leach heavy metals whether these metals are sourced from a stain or from raw metal oxides. Thus you should have your glazes tested for leaching if you are making functional ware having stained food surfaces. There are many factors that determine if a glaze is leachable.