|Monthly Tech-Tip |
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.
Getting the color you want should be quite easy. Just get a stain, throw it in a base glaze and you have it, right? Wrong. Color is unfortunately not something you can easily predict or calculate. It is easy to get 'color' but it is not easy to get 'the color' that you might be looking for at the temperature you want it at. People who have considerable experience and know-how in ceramics know about color, how to achieve it, enhance it and control it in different glaze base chemistries.
Achieving a specific color has a significant trial and error component. It is not something that is easy to test and put numbers to. There are so many trade-offs in working with ceramic colors: formulations, glazes, temperatures, atmospheres, materials, techniques, and costs.
If you are just entering the world of color control as opposed to color accidents, I recommend that you first develop a reliable and adjustable transparent glaze at the desired temperature (see the chapter Concentrate on One Good Glaze). Then begin adding colorants and adjusting the chemistry if needed.
The Digitalfire Reference Database has a Color subsection in the Properties area, it can help you investigate and achieve the desired color. For example, if you look up cobalt (in the Materials area) there are linked properties for blue that explain how to get a reliable blue and things to watch out for that may alter or hinder the blue coloration. Conversely, in the blue color section of the Properties area you will find the same properties listed, plus all the other ways to achieve blue (using oxides and other materials).
Not many things make a new ceramist more excited than the possibility of creating bright glaze colors and decoration. At the same time, few things match the frustration of trying to achieve the needed color with the right surface quality. As most of us know, metal oxides are the key to creating color. But not without some trouble. It is true, for example, that a little chrome usually gives green but the shade of green is likely not right. Further, different host glazes will produce different greens and the chemistry of some may not even give green at all. There are other problems with raw colorants too. You will frequently experience specking with powders that have not been ground fine enough and color shifts will be common with different grades or suppliers. Furthermore, the fired color seldom matches the raw color. Seeing a brown, raw glaze fire green or a gray one fire blue is a novelty that quickly wears off when you must visualize fired color when decorating.
Fortunately, stain manufacturers have addressed these problems for us. They have done the endless blending to achieve a myriad of different color shades, and they have established stable "oxide systems", which function more reliably. Many well documented oxide groupings have evolved (e.g. chrome-iron-cobalt-nickel black). Users like us have the benefit of working with more reactive, reliable, and safer materials. And since stain colors have been pre-fired, the raw and fired colors match. Admittedly, for many applications and certain easily achieved colors, the raw metal oxides are suitable and more economical. However, in most cases, when all costs are considered (including development, re-fires, etc.) and when the final quality is evaluated, the stain route can be less expensive.
Unfortunately, suppliers usually have little or no support literature to assist you in creating an effective host glaze for each stain. They traditionally employ specialists that customers can phone for advice. Unfortunately, if you are a potter, this is not really an option, as the stain company cannot possibly field questions from thousands of what they consider casual users. So, it is up to you to make the stain powders cooperate with your glazes.
After finding a supplier, obtain a selection of sample colors and if possible a master list showing each stain, its oxide make-up, and type. First, determine which stains your glaze is compatible with. Remember, stains can be suitable for glazes or bodies or both. A body stain will usually not melt properly in a glaze. The Digitalfire Reference Library documents many stains in its materials database. In addition, it outlines major stain systems and what types of applications each is suitable for.
For example here is a report on a generic stain type:
STAIN (BLUE) - CoAlZn Blues
DATE: 10/02/95 SUPPLIER: GENERIC
DETAIL NOTES Certain colors may employ controlled quantities of calcium or barium compounds as modifiers.
Color Range Dark mazarine blue through royal blue to matte blue. Generally darker and more violet in shade than zircon-vandium blues.
Specific Gravity: 3.5-4.5
Stability/Kiln Atmosphere Excellent in most types of glazes and bodies for firing in oxidizing or a slightly reducing atmosphere at temperatures over 1450C.
Compatibility Suitable for blending with most other types of ceramic colors including zircon-vanadium blues.
Cost Cost is low to moderate, increasing as the cobalt content increases.
Note: This description is taken from information provided by Blythe Matthey and may or may not apply to stains of this family from other companies.
PROPERTIES Glaze Color: Blue CoAlZn Silica based blues are fusible, tending to soften the glaze. This tendency can be overcome by the addition of china clay to the glaze.
Underglaze Color: Suitable for use under all types of glazes firing in either an oxidizing or a slightly reducing atmosphere at temperatures up to 1450°C. Cobalt-silica blues also find wide applications as "oven blues", the color being applied over the glaze and fired at the glaze firing temperature.
Body Color: Blue CoAlZn Suitable for all types of bodies and slips for firing at temperatures up to 1450°C. Not suitable for bone china bodies, since cobalt content of the blue combines with the phosphate content of the body, producing an unstable violet or reddish shade.
Courtesy Blthye Matthew Corp. information sheet.
It is best to pick just one quality transparent glaze (if you don't have one, start by taking the opacifier out of a white). If possible, use one having 10% or more CaO and low MgO and zinc. Also, it should be ultra clear, bubble and crystal free, fit your clay bodies, be adjustable in melting temperature and expansion, and use inexpensive materials. If you do not have a such a glaze, the Digitalfire Reference Library has a number for different temperature ranges in its sample recipe database.
Make a beer-mug-sized cylindrical shape out of a white clay (white clays produce the most vibrant colors). Bisque fire it, then mix each pure stain with water and paint its own unique identification numbers on the piece. Finally, dip the entire piece in the transparent glaze and fire. Repeat this process on another piece, this time painting them over the glaze. After firing, note the colors that perform well and mark them ready for use. For the ones that do not fire correctly, you have three choices:
While it may not be possible for one glaze to cooperate with all colors, it should be feasible to use one glaze and variations thereof.
Your tests will doubtless demonstrate that certain stains are temperamental. Each has its own physical personality. Some are potent, producing intense color in small amounts, while others are much weaker. Some settle quickly in water; others more slowly. Some resist being wetted by an over or underlying glaze melt; others fuse readily. Again, the key to successful color is to harmonize the chemistry of the glaze with the oxide system of the stain.
|"...certain stains are temperamental".|
Before devoting too much energy to making a glaze-stain combination work, remember that for any given color there are usually a number of oxide systems that a stain manufacturer can employ to produce it (e.g. manganese alumina pinks, chrome tin pinks). As already mentioned, if one does not cooperate with your glaze another might. As luck would have it, manufacturers compete with products in each of the common oxide systems and they advertise these as such. Thus you are never usually stuck with one manufacturer. For example, all of them make one or more chrome-tin pinks, so any information discovered about using this system applies to the products of all companies. Similarly, you will find that while the Digitalfire Reference Library may not document a stain from one manufacturer, a stain of the same type from another may have the information you need.
You can achieve the color by putting stain in, over, or under a glaze. If you are simply aiming for a colored glaze, then, depending on the potency of the stain and the desired intensity, just add 1%-10% to the glaze batch. However, you may discover that your crystal clear transparent glaze with added stain produces a variegated or crystalline surface that varies with glaze thickness and firing. With some stains it may be necessary to adjust the glaze to achieve more even color or ball milling may improve the situation.
Using stains for colorful brushwork decoration is very popular, and many people just mix the pure stain powder with water and paint it on using watercolor techniques. In general, a stain should be used under a transparent glaze at higher temperatures where edges tend to feather and smear with overglaze techniques. The ware can be decorated at the green or biscuit stage, and colors will fire are softer. However, under-glaze stains must have maximum chemical compatibility with the overlying glaze for correct color development. If a stain is used over a glaze, colors are more vibrant, edges of colored areas are softer, and glaze crawling due to improper wetting of the underlying stain is reduced. Note that the solvent action of fritted glazes is diminished because of the fritting process; therefore these will produce less smudging of colors and bleeding edges.
Whether over or under glaze, it is generally better to mix stains with mediums to both extend and condition them. This is because there are a number of problems associated with using a pure stain-water mix for brushwork.
Thus, a medium should extend the expensive stain and impart suspending, melting, drying retardant, and hardening properties while having a sympathetic chemistry and melt stability for the color (for use under glaze hardener is not necessary). In general, the use of some kaolin or bentonite in the medium mix will impart suspending, retarding, and hardening properties as well as supply Al2O3 to stiffen the melt. For overglaze colors, it will be an advantage to add a gum or resin to make the mix paintable and more durable for handling. A melting agent could be as simple as a general purpose borax frit for low-temperature ware (some function much better than others) or a smooth transparent glaze for stoneware (it is, of course, possible to adjust the character of the color by using a matte or glossy medium). In theory, only enough flux should be added to achieve the required melting action and make the stain collaborate with the glaze. Since some stains will melt to a different extent in the medium than others, tuning the amount of medium to each becomes a way to "level the playing field" as it were. A medium can be tuned to fuse at a particular temperature and its make-up can be adapted to any stain or application desired. The medium can even isolate a stain somewhat from a glaze whose chemistry might be hostile to proper color development. Also, a finely tuned body medium/stain mix can be used on an unglazed body surface to achieve very subtle textured colors. Such a mix can even be applied to a leather hard piece for subsequent incising through to the body below.
If you do not have a glaze recipe that is compatible with most stains, consult the Digitalfire Reference Library for both a recipe and information on the glaze chemistry requirements of the stains you want to use. You can then formulate the medium to work with the desired stains.
Stain-medium mix is shrinking less than leather-ware and flaking off:
Stain-medium mix is shrinking more than the ware (producing a crack network of islands with curled edges):
Stain decoration is dusting and smudging during handling of ware:
Uneven coverage of stain:
Fired glaze surface is bubbling or wrinkling:
Glaze crawls away from underlying stain:
Stain color is muddy or gray:
White spots on glaze that is over a stain or stain/medium mix:
Stained section of glaze is shiny while other areas are matte, or vice versa:
Does not paint well and dries too fast:
Color of stained surfaces is not consistent:
An Overview of Ceramic Stains
Understanding the advantages of disadvantages of stains vs. oxide colors is the key to choosing the best approach
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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