Monday, April 20, 2009
Earthenware Glaze Tests and Recipes
I have just unloaded my electric kiln which had a smallish load of glazed earthenware pots. The firing was to cone 03 well down, possibly nearer cone 02 (about 1100 degrees centigrade).
Pots made by children and adults on the demonstration day.
Most of the reason for the firing was to glaze fire 12 small pots that people made at my demonstration day at Waitati three weekends ago.
I was quite pleased, and relieved, with how they turned out.
I glazed the 12 small pots with a glaze that I have slightly modified from a recipe by Janet DeBoos.
One of my little bowls
At cone 03-02 it will give an aqua green with a nice iron red break in oxidation over terracotta clay,
Wood fired terracotta planter with copper red version of this glaze.
and a good copper red in reduction. It also fits reasonably well, but not perfectly.
Glaze number 17 in More Glazes For Australian Potters by Janet DeBoos is for cone 05-03
Fritt 4124 80
Copper carbonate 6
I have changed it slightly as follows
Fritt 4124 80
China Clay 5
Tin Oxide 1.5
copper carbonate 2
This raises the maturing temperature and the amounts of tin and copper make it suitable for copper reds.
Using Petalite introduces lithium carbonate, and I suspect helps give the alkaline blue-green colour to the glaze.
An 8 1/4 inch bowl by me with the modified DeBoos glaze. Note the feathering of the edges of the cobalt decoration where there was some movement.
I line tested my DeBoos modified glaze base, minus the tin and copper, with other metal oxides added.
The base had a curious response to iron, remaining resolutely honey coloured up to 12 to 16 % iron.
I tested a couple of small pots with the base plus 8% iron, and got quite a toothsome looking rustic glaze, that I mistook at first for one of my lead bi-silicate honey glazes.
I also fired some test tiles and other glaze testers.
Lead bi-silicate= 60, Potash Feldspar= 10, whiting= 5, China Clay= 15, Silica= 10, plus Red Iron Oxide= 2.5. (I think this is an Emanuel Cooper Glaze, or is from one of his books). This gives a satin honey coloured glaze, that is crazing and slightly sugary. All metal oxides are dark and dull over it. I wonder if it needs more heat to improve clarity and fit?
Lead bi-silicate= 100, Red Clay= 22, China Clay= 10, Tin Oxide= 12, gives a silky semi-gloss ivory white with attractive pink where thin over terracotta. Red brown response to iron, and manganese, copper, and cobalt all slightly and pleasantly quietened. Glaze a good fit so far.
Frit 4124= 90, China clay= 10, bentonite= 3, tin oxide= 7, and have a satin to full gloss white that is pale pink over terracotta, and over the white slip on the tile. Iron is straw coloured over it, manganese dull greyish brown, copper good, and cobalt slightly restrained. The glaze is fitting well 4 hours after firing.
Frit 4110= 90, China Clay= 10, plus zirconium silicate= 12, gives a translucent white over terracotta, and a good white over white slip. The glaze has a green response to iron, plummy response to manganese, blue-green response to copper, and cobalt is strong and rich like stained glass. The glaze is a poor fit, crazing as soon as out of the kiln, but would stain as an attractive crackle.
Lead bi-silicate= 80, Red Clay= 20, Tin Oxide= 7, Copper carbonate= 2. This needs to be moderately thick to cover well at this temperature. An attractive creamy green gloss glaze, with red break where thin over terracotta clay. Good fit so far.
This is a little test pot with the lead/tin white glaze that is the same as the second test tile, the floral decoration was to test the oxide response. Manganese around the rim, small brown "flowers" are red iron oxide, green is copper carbonate, and blue is cobalt carbonate.