|Monthly Tech-Tip |
Understanding the advantages of disadvantages of stains vs. oxide colors is the key to choosing the best approach
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 stain company that provides good service are key factor to being able to troubleshoot color problems with 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. Still, variegating agents can be added (like titanium and rutile).
Potters love to paint stains over and under glazes to decorate ware. Majolica ware is a good example. However, stains are refractory, they resist allowing overlying glazes to envelope the particles and then fasten onto the underlying body. Stains used for underglaze decoration, for example, need to be mixed with a recipe of materials into which they can melt and suspend, one that melts enough to attach to the body but not so much so as to bleed excessively into the overlying glaze. Different stain types require mediums of different chemistry, ones that enable the color development and have the proper degree-of-melt. Some stains 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 something that will suspend the particles, slow down the drying and harden when dried.
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. Stains are made from the same metal oxides you would use to get the color, they have simply been prefired and blended for specific colors and often mixed with other materials to make them melt higher or leach less. Thus you should have your glazes tested for leaching if you are making functional ware having stained food surfaces. Better yet, use a liner glaze. There are many factors that determine if a glaze is leachable (for more information see the links on this page).
On the left is a pure blue stain, on the right a green one. Obviously, the green is much more refractory. On the other hand, the green just sits on the surface as a dry, unmelted layer. For this type of work, stains need to be mixed into a glaze-like recipe of compatible chemistry (a medium) to create a good, paintable color. The blue is powerful, it would only need to comprise 5-10% of the recipe total. Its medium would need to have a stiffer melt (so the cobalt fluxes it to the desired degree of melt fluidity). A higher percentage of the green stain is needed, perhaps double. It's medium needs much more melt fluidity since the stain is refractory. Of course, only repeated testing would get them just right. Guidelines of the stain manufacturer for chemistry compatibility need to be consulted also (as certain stains will not develop their color unless their glaze medium host has a compatible chemistry). And, to be as paintable as possible, use use a gum-solution/water mix (e.g. 2 parts water to one part gum solution).
Mason Color Reference Guide
Ceramic stains are manufactured powders. They are used as an alternative to employing metal oxide powders and have many advantages.
Glaze and Body Pigments and Stains in the Ceramic Tile Industry
A complete discussion of how ceramic pigments and stains are manufactured and used in the tile industry. It includes theory, types, colors, opacification, processing, particles size, testing information.
Getting the Glaze Color You Want: Working With Stains
There are many things to know about to make the best use of stains, but one often ignored aspect is the relationship between glaze color and chemistry. If you want to control color you need to know about stains and chemistry.
Is Your Fired Ware Safe?
Glazed ware can be a safety hazard to end users because it may leach metals into food and drink, it could harbor bacteria and it could flake of in knife-edged pieces.
Having Your Glaze Tested for Toxic Metal Release
Having Your Glaze Tested for Metal Release
Are Your Glazes Food Safe or are They Leachable?
Many potters do not think about leaching, but times are changing. What is the chemistry of stability? There are simple ways to check for leaching, and fix crazing.
|Oxides||MnO2 - Manganese Dioxide|
G2853B - Cone 04 Clear Ravenscrag School Glaze
A Ravenscrag Slip base made by simply mixing it 50:50 with a frit
|By Tony Hansen|
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