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Stain, Pigment


Stains are man-made colored powders used in glazes, bodies and engobes. They are manufactured by sintering powdered components in special furnaces at high temperatures. The powdered mixtures are stoichiometric, carefully compounded, finely ground and vigorously blended so that adjacent particles react without needing to be melted. After firing the material is ground in such a way as to control the particle size within a specific range (often unique to each type of stain). Some stains are also acid-washed after grinding. These processes render them more resistant to dissolving in glaze melts or melting themselves compared to the metal oxides from which they are made. Different types of stains have differing levels of stability against temperature. For many colors there are a variety of stain chemistries that can produce them, each has advantages and disadvantages. Some colors, however, can only be made using one specific chemistry and/or process.

Theoretically, since stains are inert any stain should develop its color well in any transparent base glaze. Unfortunately, this is not the case in actual practice. There is enough interaction with the host glaze that many stains require a specific chemistry in the host glass to develop the color. Chrome-tin pinks are an example. Stain manufacturers classify the idiosyncrasies of the colors they produce and document the cautions of each class. In this way glaze, underglaze and overglaze manufacturers can tune their products to work.

It is important to realize that stains are not to be employed pure. They are used in-glaze, as a percentage. This can be as little as 1 or 2% in some applications, but more commonly would be much more. A common mistake is using pure stains as an under or over glaze. Since they are relatively refractory, if you spray stains on pure and put glaze over that, the glaze cannot bond. It is like painting a dirty car. The stains have to be mixed with a melt medium that envelops the particles and bonds with the body below and glaze above. But the medium must not melt too much or colors will bleed into the over or under lying glaze. The medium also needs a hardener so it dries durable enough to handle. If the consistency is a liquid (rather than paste) it also needs a suspender so particles do not settle out in the slurry. Each stain needs a medium that will not be hostile to its color development, that means you cannot use a one-medium-fits-all-stains approach. Also, some stains melt more and some less, so the medium needs to take this into account. A melt flow test should be done to fine tune the flow of each (see link below).

Pictures

Organized testing of stains in a base recipe

A quick and organized method of testing many different stains in a base glaze: Prepare your work area like this. Measure the water content of the base glaze as a percent (weight, dry it, weight it again: %=wet-dry/wet*100). Apply labels to the jars (bottom) showing the host glaze name, stain number and percentage added. Counterbalance a jar on the scale, fill it to the desired depth, note the amount of glaze and calculate the weight of dry powder that is present in the jar (from the above %). For each jar (bottom) multiply the percent of stain needed by the dry glaze weight / 100. Then weigh that and add to the jar and put the lid back on. Let them sit for a while, then shake and mix each (I use an Oster kitchen mixer). Then dip the samples, write the needed info on them and fire.

Mason stains in G2934 matte base glaze

These are Mason stains added to the cone 6 G2934 silky MgO matte liner base glaze (with tin, zircopax and various stains added). The brightest colors (6600, 6350, 6300, 6021, 6404) were tested overnight in lemon juice without visible changes. It is known that MgO mattes are less prone to acid attack that CaO mattes. Caution is required with inclusion stains (like #6021), if they are rated to cone 8 they may already begin bubbling at cone 6 is some host glazes.

Each stain has its own personality for coloring the body

All of these Mason stains make the porcelain more refractory, but some more so (e.g. 6385, 6226). Some do not develop the intended color (e.g. 6006 pink). Some need a higher concentration (e.g. 6121, 6385). Some need a lower concentration (e.g. 6134). Some do not impart a homogeneous color (e.g. 6385).

Cone 6 porcelain marbled and thrown

These bowls were made by Tony Hansen using a mixture of white and stained New-Zealand-kaolin-based porcelain (Plainsman Polar Ice) fired at cone 6. The body is not only white, but very translucent.

What happens when you opacity a colored glaze?

Left: G2934 cone 6 matte glaze with 3% Mason 6300 blue stain. Right: An additional 4% tin added. Notice how an opacified color does not have depth and therefore is lighter in color. Also it does not break to different shades at the edges of contours the way the transparent color does.

Two stains. 4 colors. Will the guilty oxide please step forward.

We are looking at two pairs of samples, they demonstrate why knowing about glaze chemistry can be so important. Both pairs are the same glazes: G2934 cone 6 matte and G2916F cone 6 glossy. The left pair has 5% maroon stain added, the right pair 5% purple stain. The red and purple develop correctly in the glossy but not the matte. Why? The Mason Colorworks reference guide has the same precaution for both stains: the host glaze must be zincless and have 6.7-8.4% CaO (this is a little unclear, it is actually expressing a minimum, the more the CaO the better). The left-most samples of each pair here have 11% CaO, the right-most have 9%. So there is enough CaO. The problem is MgO (it is the mechanism of the matteness in the left two), it impedes the development of both colors. When you talk to tech support at any stain company, as I did with Mason on this, they need to know the chemistry of your glaze to help, not the recipe.

Mason stains in a cone 6 clear base

These are Mason stains added to cone 6 G2916F clear liner base glaze. Notice that all of these stains develop the correct colors with this base (except for manganese alumina pink 6020). However caution is required with inclusion stains (like #6021), if they are rated to cone 8 they may already begin bubbling at cone 6 is some host glazes.

Mason stains in a cone 6 clear base

These are Mason stains added to cone 6 G2926B clear liner base glaze. Notice that the chrome tin maroon 6006 does not develop as well as the G2916F glossy base recipe. The 6020 manganese alumina pink is also not developing. Caution is required with inclusion stains (like #6021), if they are rated to cone 8 they may already begin bubbling at cone 6 is some host glazes.

Stains having varying fluxing effects on a host glaze

Plainsman M340 Transparent liner with various stains added (cone 6). These bubbles were fired on a bed of alumina powder, so they flattened more freely according to melt flow. You can see which stains flux the glaze more by which bubbles have flattened more. The deep blue and browns have flowed the most, the manganese alumina pink the least. This knowledge could be applied when mixing these glazes, compensating the degree of melt of the host accordingly.

Two different shipments of a cobalt free black stain. Why different colors?

This is not actually bad, it is good. Stain companies make adjustments when they receive shipment of off-standard raw colorants, this insulates the end user from fired variations in color. In this case, they added extra chrome (to the one on the left), the final product produces the same colored black glaze.

Maroon and white mug before and after firing: What a difference!

The outer glaze is Ravenscrag GR6-E Raspberry, the bright maroon color is a product of the surprising interaction between the 0.5% chrome oxide and 7.5% tin oxide present. That small amount of chrome is only enough to give the raw powder a slight greenish hue, hardly different than the clear liner. While this color mechanism appears to be effective, it is delicate. A maroon stain is actually a better choice. It would fire more consistent would be less hazardous to use. And the raw glaze will be the same color as the fired one!

Why you cannot paint pure stain powders over glaze

An example of why you should not just paint pure stain powders over glazes. Left is a blue stain, right is a green. Obviously the blue is melting in much better, even bleeding at its edges. On the other hand, the green just sits on the surface as a dry, unmelted layer. Stains need to be mixed into a glaze-like recipe of compatible chemistry (a melt medium). The blue is powerful, it would only need to comprise 5-10% of the total, the green 10%-15%. Overglaze recipe development projects involve following the guidelines of the stain manufacturer for chemistry compatibility and adjusting the melt to compensate for each stains melting behavior.

How to include stains in chemistry calculations in Insight

The simple answer is that you should not. The chemistry of stains is proprietary. Stain particles do not dissolve into the glaze melt like other materials, they suspend in the transparent glass to color it. That is why stains are color stable and dependable. In addition, their percentage in the recipe, not the formula, is the predictor of their effect on the fired glaze. Of course they do impose effects on the thermal expansion, melt fluidity, etc., but these must be rationalized by experience and testing. But you can still enter stains into Insight recipes. Consider adding the stains you use to your private materials database (for costing purposes for example).

Out Bound Links

In Bound Links

  • (Project) Stains

    We make no attempt to classify or compile stains a...

  • (Glossary) Jasper Ware

    A porcelain manufacturing technique developed by E...


By Tony Hansen




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