This is a technique originally used to seal low fire pottery. It is believed to have originated in Eastern Europe during the Middle Ages. There is a revival of the process underway in Ukraine and the Baltics. It is nicknamed "Baltic Raku". The solution which I call the "brew" is a mixture of flour , yeast and water. See the recipe below. It is mixed three days in advance of firing. Pieces are made and bisqued before the final firing. Using a raku kiln, fire a batch of pots to 1650 degrees F. Pieces are removed one at time, dunked into the brew and then quickly into water. Then they air cool. The brew burns quickly on the surface of the piece before going into the water. The water stops the burning and color change.
If the kiln gets below 1500 before al the pots are processed, turn the kiln back on and take it back to 1650 F. It the piece is too cool the process doesn't work and makes a slimy mess.
Photos show the loaded kiln, temperature reached, stir the brew, dip the hot pot, empty the brew out of the pot, dip into water, empty the water, set to cool. There is no smoke, only steam. The last pot shows lichen pattens on a smooth burnished surface.
I use Jane' Jermyn's recipe for the brew.
Jane Jermyn’s Obvara Recipe
2.2 pounds of flour
1-2 packets of yeast
1 Table spoon of sugar
2.6 gallons of water
Mix well, cover and let it ferment for 3 days. Stir several times daily and keep the brew warm. It will smell like stale beer.