Cobalt is the only common colorant that behaves the same in every atmosphere, at every temperature, and in every base recipe. That reliability makes it the best colour to learn glaze chemistry with, and its weakness: cobalt is a strong colourant, so 1 percent is already a deep blue and 3 percent is nearly black. This is a low-fire base built to take 0.6 percent and give a clear, stable blue.
H 10 · Ø11 / Ø12 / Ø6 cm
Base
A low-fire transparent base for cone 04 in oxidation. Built on frit rather than on feldspar, because at 1060 degrees Celsius a feldspathic base never fully dissolves and stays milky.
| Material | Parts | Share |
|---|---|---|
| Frit 3124 | 40 | 42.7% |
| Silica (SiO₂) | 20 | 21.4% |
| Whiting (CaCO₃) | 15 | 16.0% |
| EPK Kaolin | 15 | 16.0% |
| Bentonite | 3 | 3.2% |
| Cobalt Carbonate (CoCO₃) | 0.6 | 0.6% |
| Total charge | 93.6 |
Fired surface
A deep, slightly purple blue with a glassy surface. At 0.6 percent cobalt the colour is a strong mid-blue; at 0.3 percent it is a soft powder blue. The colour is completely stable from one firing to the next.
Why frit and not feldspar at low fire
At cone 04, 1060 degrees Celsius, there is not enough heat to dissolve feldspar. A feldspathic low-fire glaze fires to a milky, under-melted layer that scratches easily and looks like a bad enamel. Frit 3124 is already a glass, so it melts at the temperature the kiln actually reaches and gives a clear, hard surface.
The trade-off is cost: frit is four to five times the price of feldspar by weight. For low-fire work the clarity is worth it; for anything above cone 2, use feldspar and save the money.
Cobalt is strong: measure it properly
Cobalt carbonate is about 45 percent cobalt by weight, so 0.6 percent of the carbonate is roughly 0.27 percent cobalt oxide. That is already a solid mid-blue. If you are used to recipes that call for 2 or 3 percent cobalt, they are producing a nearly black glaze and probably also a lot of pinholing, because cobalt is a strong flux at low temperature.
Weigh cobalt to 0.01 g and mix it into a small amount of water before adding it to the bucket. Cobalt carbonate does not disperse well when added dry to a large volume of glaze; it forms blue streaks that fire as visible lines on the pot.
The bentonite and the settling problem
Low-fire frit glazes settle like stone in a bucket. The 3 percent bentonite in this recipe is there to keep them in suspension, which it does for about four hours. For longer working time, add 0.2 percent Epsom salts as well, and stir the bucket with a drill for two minutes before every session.
A settled low-fire glaze is dangerous to use without full mixing: the top of the bucket is thin clear glaze and the bottom is a cobalt-rich sludge. Dipping without mixing gives a pale pot the first day and a nearly black one the next, from the same bucket.
Using it over decoration
This base is clear, so it can be used as the transparent layer over slips, underglaze pencils and oxide washes. At cone 04 it is gentle enough that iron and copper washes stay where they were painted instead of bleeding, which makes it a good topcoat for detailed decoration on earthenware.
Applied over a red earthenware body it goes slightly green where it pools, because the calcium in the glaze picks up iron from the body. On white earthenware, the cobalt version fires the same blue every time, which is why it is the standard colour for repeat production work.
Steps
- Weigh the five base materials and the bentonite, and combine them dry.
- Dissolve the cobalt carbonate in a cup of water and add it to the mixing water.
- Add the base materials to the water and mix for five minutes with a drill and paddle.
- Measure the specific gravity and adjust to 1.40, which is right for a low-fire dip glaze.
- Sieve through 80 mesh, then add 0.2 percent Epsom salts if the glaze settles within four hours.
- Dip test tiles for one second and check the blue against a fired reference tile.
- Stir the bucket with a drill for two minutes before every dipping session without exception.
- Fire to cone 04 in oxidation with the kiln vented until 600 degrees.
Where it goes wrong
- Adding cobalt carbonate dry to the bucket. It forms streaks that fire as visible blue lines.
- Using a feldspathic base at cone 04. It fires milky and soft no matter how long you soak.
- Dipping without stirring. The first pot of the day and the last are two different blues.
- Substituting cobalt oxide for cobalt carbonate by the same weight. The oxide is twice as strong.
| Symptom | Cause |
|---|---|
| Blue streaks on the fired pot | cobalt was added dry and never dispersed; dissolve it in water first. |
| Glaze is milky rather than clear | the base is feldspathic or the kiln did not reach cone 04. |
| Colour varies between pots dipped on the same day | the bucket was not stirred fully before each dip. |
| Pinholes concentrated where the blue is darkest | cobalt level above 1 percent, which over-fluxes the glaze at low fire. |
Recipe and schedule numbers here were recorded on the workshop’s own kiln. A different kiln, a different thermocouple position or a different clay batch will move the result, so treat every firing schedule as a starting point and keep your own log beside it.