All Glossary

Stain Medium

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: powder medium, stain medium - Learn more

Details

Ceramic stains are highly concentrated refractory pigment powders. Unlike pure metal oxides, they do not generate gases on firing, color is more stable, they are less toxic and they do not melt. Stains also increase color in a more linear fashion according to percentage. But, stains have no ceramic or working properties, so DIY potters must add them to host glazes, engobes, underglazes or slips that do (these are thus “stain carriers”).

Consider first 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.

Consider first why stains need a Ceramic medium. Glazes fire as they do because they have an SiO₂:Al₂O₃:flux chemistry tuned to melt at the desired temperature, to the right surface character and required thermal expansion. Stain powders alone impart none of this. Fortunately, 1-10% stain will often pigment a glaze, no compensatory recipe change is needed. However, engobes, slips and underglazes don’t develop a glass, so they need a far higher percentage of pigment. And they also need high percentages of plastic clay to make them sticky and bond well with the unfired body.

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 (obviously, a water based glaze cannot be applied over this with firing it on). For Cuerda Seca, an oil-based ink is used for the express purpose of resisting the glaze.

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.

The performance, or lack thereof, of some commercial underglazes motivates many to develop their own. For example, some commercial products develop either too much or too little in firing. Some intended for painting don't paint well! Others don't accept overglaze very well. Others don't silk screen well. Using the principles above you can develop a carrier that imparts the right working and ceramic properties.

Related Information

Why it is not a good idea to use straight stain underglaze

This picture has its own page with more detail, click here to see it.

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, from that it inherits better lays down, accepts the overglaze better and dries harder. Black stains are potent, an 80:20 stain:glaze mix would work even better.

Silk screen mediums

This picture has its own page with more detail, click here to see it.

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).

Here is another reason never to use only a stain-water mix for pottery decoration

This picture has its own page with more detail, click here to see it.

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.

A Clear Glaze is Bubbled Over DIY Underglazes:

The Real Solution Is in the Underglaze Recipe

This picture has its own page with more detail, click here to see it.

Potters often encounter the problem shown here. These pieces are fired at cone 6. They are decorated with underglazes made from a mix of porcelain powders and stains. The transparent glaze works over certain colors but on others, it is full of microbubbles and pinholes. The potter has not had success finding a transparent overglaze that works consistently. Stain manufacturers do not mix stains with porcelain to making underglazes.

So, although closer control of the transparent glaze thickness or a more fluid melt glaze recipe might help, the real solution may lie with the underglaze recipes used here. An ideal bisque-stage underglaze is sinter-bonded but not sealed (therefore not accepting glaze water). An ideal fired underglaze also has controlled maturity: enough glass development to bond well to the body and promote glaze acceptance, but not so much that edge-bleeding and opacity loss occur. This state of 'controlled maturity' is also more likely to match body thermal expansion. The cost savings and the potential to fine-tune each color to your exact needs can be powerful motivations to use DIY underglazes.

Faux majolica next level: Stoneware!

But the glaze is crawling under the colors.

This picture has its own page with more detail, click here to see it.

The original Italian majolica ware was red earthenware with a thick layer of tin-opacified glaze vibrantly brush-decorated using single-strokes of watery metal oxides. The water-color of ceramics. But tin oxide is no longer affordable. And ceramic stains are better. And no one uses lead glazes. So all majolica-like ware made today is actually “faux (false) majolica”. These test samples take the “faux” to the next level: Stoneware with a zircon-opacified white glaze. But almost all are crawling. If this happens for you ask these questions:

Is the glaze re-wetable? Dipping glaze recipes often are not, especially if they fail sanity check (e.g. are over-clayed or under-clayed).
Base coat dipping glaze better survive the rewetting of a second layer?
Mixing them as a brushing glaze give maximum insurance.

What did they look like when the overcolor dried? Cracks are sure indicator or crawling.
Were you painting pure stain or metal oxide (mixing with water only)? Don’t do that. Water color paint uses gum Arabic, pottery colors need to be in a stain medium (which often has CMC gum).

Inbound Photo Links

How can underglaze and engobe colors be this bright?

Links

By Tony Hansen Follow me on

Got a Question?

Buy me a coffee and we can talk