|Monthly Tech-Tip |
It is a mistake to use pure stains for decorating ware. Stains need to be mixed with a ceramic medium and a working medium to enable effect use and produce a good fired result.
A “stain medium” is a colorless ink or carrier to which a ceramic stain is added. It needs to function as both a ceramic medium (do what is needed in the kiln) and a working medium (apply and stay in the manner needed).
Ceramic glaze stains are non-melting, highly concentrated pigment powders. They have one purpose: impart colour in a more stable and effective manner than pure metal oxides (do not generate gases on firing, color is stable, less toxic, do not melt). They are most at home when simply being mixed, as a percentage, into a glaze recipe. Some stains will pigment a glaze with as little as 1%, others need as much is 20%. Ceramic inks, that need to be stamped or silkscreened, need other properties then just color.
Ceramic glazes typically contain clay (which suspends the slurry and hardens it as it dries), silica (the low thermal expansion glass base) and melters that fuse it all together. Stains powders alone contain none of these. They are not sticky to adhere to a surface, they don’t dry hard enough to stay on that surface or resist handling, two glass bond to a surface below or a glaze above. As a bare minimum, of a pure stain is mixed with Glycerine or some sort of oil (instead of water) and applied in a thin enough layer it could be serviceable, at least for application and handling purposes. But since stain powders are concentrated, it makes sense to mix them with a transparent glaze base. In most cases, the highest percentage of stain needed would be about 25%. That would mean that most of the beneficial properties of the glaze would carry over to the ink.
Once the ceramic recipe of the ink, that is, what it will do during during, is established, then the working recipe needs to be determined. Depending on the method of application of the ink (e.g. stamping, silk screening, brush work, cuerda seca) it would be either oil-based or water-based. For the latter it would typically be a combination of water and some sort of hardener (e.g. silk screen medium). For stamping it could be simply a powder glycerine mix. For brushing it would be a water and gum mix. The medium for silk screening would depend on whether designs are being screened onto transfer paper or directly onto a ceramic surface. Some research and a little common sense will generally suffice to develop a working medium for the purpose you need.
Most application methods are obvious but many people wonder how to rubber-stamp an ink. The simplest method is to simply mix the powder with enough glycerine to make a thick paste. This is quite easy to do, put some glycerine on a piece of glass and just keep stirring in powder (using a pallet knife or similar tool) until the right viscosity is achieved. Then, using a rubber roller, roll out a thin layer of the paste onto a piece of glass. Press the stamp onto that layer to ink it, then press it onto the ware. Experimentation will quickly reveal the best consistency for the ink.
If glaze is being applied over an ink there are issues with adherence. For cuerda seca, an oil based ink it used for the express purpose that the glaze does not stick. In stamping, glycerine works well because it does not dry. Glaze will adhere, but does so in a thinner layer because body absorbency is unable to pull water through it. This actually turns out to be an advantage, since the thinner layer of transparent conveys the color better (having fewer micro bubbles). This phenomenon underscores the need for the stain to be mixed with a ceramic medium, one that bonds well with the overglaze. For silk screening inks for printing onto transfer paper there are other issues. The ink must dry on the paper to enable storage of prints without smearing them. On wetting the paper it must release the ink, which in turn, must adhere to the ceramic surface. These demands make it best to mix a professional silk screen working medium with your stain-mixed-with-ceramic-medium powder.
Is it better to make your own medium or buy a premixed ink or underglaze? Unless you try products from multiple manufacturers and test them with a critical eye, it may be better to make your own. Some commercial products melt either too much or too little, indicating issues with the ceramic medium they are using. Some manufacturers are unaware of even the need for a ceramic medium. Some products intended for painting don't paint well! Some don't accept overglaze well. Others don't apply well, this is especially so for silk screening. There are so many applications for ceramic inks that one product obviously cannot satisfy the needs of all. Since the appearance of your ware is at stake it does make sense to learn to mix your own colors and optimize them to your process. That means that for the same color you will have multiple mixes that adapt to each melt of application.
The logo on the left was rubber-stamped using and ink mix made of only glycerine and Mason 6666 black stain. The glaze is shedding off during firing. Multiple properties needed by a stamping ink are not present here. First, the stain dries as a powder, it has no hardening or bonding properties, glycerine is its only mechanism. Second, it is too concentrated, the black color is so powerful that it bleeds excessively into the overlying glaze. Third, it does not melt during firing so it does not bond with the body below. And, it either develops only a fragile interface with the glaze above, or sheds it off. The piece on the right mixes the stain 50:50 with a glossy transparent glaze (having 20% kaolin), it lays down better, accepts the overglaze layer better (because it has less glycerine), presents less problems in handling before glazing and it has no issues with the overglaze crawling off during firing. Black stains are potent, a 75:25 stain:glaze mix would work even better.
Blend the two types, permanent and washable, with a powdered colorant, in the proportions appropriate to get as much hardness as possible but not so much that it is difficult to clean up the screen. The powder should be a ceramic stain mix with a melter medium (a glaze or frit).
This is a type of stain manufacture that enables the use of metal oxides (like cadmium) under temperature conditions in which they would normally fail.
Ceramic stains are manufactured powders. They are used as an alternative to employing metal oxide powders and have many advantages.