|Monthly Tech-Tip |
It is a mistake to use pure stains for decorating ware. Stains need to be mixed with a ceramic carrier and a working medium to work and fire well.
Key phrases linking here: stain medium - Learn more
Ceramic stains are highly concentrated refractory pigment powders. Since stains have no ceramic or working properties they must be mixed into a carrier or host. Yet stains are better than pure metal oxides. They do not generate gases on firing, color is more stable, they are less toxic and they do not melt. Stains increase color in a linear fashion according to the percentage used. Stains are most at home when mixed, as a percentage, into a host glaze recipe, engobe, underglaze or slip (a ceramic carrier).
Consider why stains need a ceramic carrier: Ceramic glazes fire as a glaze because they have an SiO2:Al2O3:Flux chemistry tuned to melt at the desired temperature, surface character and thermal expansion. Stain powders alone impart none of this. Some stains will pigment a glossy glaze with as little as 1% of the total, others might need 20% (generally both of these additions will require no change to the glaze recipe making up the other 99 or 80%). Engobes are tuned to be like the body, having similar fired shrinkage and degree of vitrification. They also contain high percentages of plastic clay to make them sticky and bond well with the unfired body. Stains don't have this. But stains are concentrated, which means they can supply color to a ceramic carrier. Where stain percentages are high more plastic clays, even bentonites can be employed, this reduces clay percentage enabling raising the silica and fluxes. As little as 5% of some stains might color an engobe whereas 50% or more might be needed in a bright underglaze (obviously, for the latter, the 50% ceramic carrier needs to be much more plastic and vitreous to counterbalance the non-plastic and refractory frit).
Consider why stains need a working medium: Brushing glazes or underglazes, for example, dry slowly because they contain gums (e.g. CMC, Veegum). Slow drying enables even coverage at any thickness (depending on coats applied and specific gravity of the slurry). It also enables and application of brushwork decoration. The gums also harden the dried surface. Glazes contain clays that keep the slurry in suspension. Stain powders alone have none of these properties.
When a stain:carrier is to be silkscreened, stamped, inkjet printed, pen applied, etc. it is considered to be an ink. Inks may be oil-based or water-based. For stamping a simple powder:glycerine mix works well, even a pure stain can be stamped because the glycerine does not actually dry. Silkscreen inks can be just a combination of water, a gelling agent and a gum hardener (silkscreen mediums are available as a liquid product). When designs are being screened onto transfer paper the ink must dry hard enough on the paper to enable storage of prints without smearing them. But, on wetting the paper, the ink must soften and release quickly and adhere to the ceramic surface. These demands require fine-tuning the hardener percentage (or using a professional silk screen medium). Screening directly onto a ceramic surface would likely require an oil medium. One more issue: When glaze is being applied over an ink there can be issues with adherence.
A procedure for rubber-stamping designs can be a challenge. The simplest method is to simply mix the powder with enough glycerine to make a thick paste (they mix readily). Put some glycerine on a piece of glass and keep stirring in powder (using a pallet knife or similar tool) until the right consistency is achieved. Then, using a rubber roller, roll out a thin layer of the paste and press the stamp onto that layer to ink it. Experimentation will reveal the best consistency for the ink.
For Cuerda Seca, an oil-based ink is used for the express purpose of resisting the glaze.
The performance of some commercial underglazes speaks to the importance of tuning the above-mentioned ceramic and working properties of your own recipes. Some commercial products melt either too much or too little. Some intended for painting don't paint well! Some don't accept overglaze well. Others don't silk screen well. That being said, some people who are doing home-grown brightly colored engobes or underglazes are paying less than needed attention to the ceramic carrier and producing pieces that are less durable or prone to flaking (because of being improperly bonded to the body). Brightly colored glazes are also being made that are prone to leaching and staining.
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).
A stain-water mix was painted into recesses and an overglaze was applied. This stain is refractory, thus it does not glass-bond with the body. And it repels being wetted by the molten glaze. Thus this problem. The stain needs to be mixed with a stain medium that both supplies a bonding glass and a clay to suspend the slurry and dry harden it. One solution is an addition of Gerstley Borate (enough to melt it to the needed degree) and extra bentonite if needed (to slow the drying time for painting). At a minimum, if the color does repel the glaze a little at least it will be melted.
How can underglaze and engobe colors be this bright?
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.
|By Tony Hansen|
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