Monday, 21 November 2011

Frustratingly frothy glazes!

The last few days have been fun and games, trying to make larger batches of the Hamada and Leach clear glazes, the main problem being that both glazes contain little or no clay. The Hamada glaze has no clay at all and consists mainly of feldspar, which means it settles out incredibly needs a helping of bentonite and epsom salts (many thanks to John Britt for his helpful advice on that!) to stop it deflocculating into an immovable lump at the bottom of the bucket. The Leach glaze contains only 10% china's basically the Hamada glaze with the addition of a little clay.

Another problem with these glazes is that after mixing with water, they froth up tremendously, so when you dip a pot in it, the glaze surface is left covered in bubbles and craters, large and small. I waited a full 24 hours for the bubbles in the Hamada glaze to subside, but it was just as bad the next day. I was thinking that it might be the rough surface of the plastic container which was causing the frothing, so i transferred it to a glass bowl and this seemed to calm things down a bit. The strange thing is that by day three, both glazes had settled down and didn't bubble up nearly as much when stirred..why this improvement happens I have no idea, but the lesson is to try to mix up glazes several days before I need to use them.

And the final issue I faced was heavy drips and runs forming on the rims of pots after dipping them. With a clear glaze this can cause problems because thicker areas may fire white or milky rather than clear. After glazing a couple of pots, I suddenly remembered that I'd bisqued them at a slightly lower temperature than the previous batch (1020 rather than 1050 degrees C) and it's surprising how much difference that makes to the speed of water absorption. Effectively, the glaze was now too thick for this lower temperature bisque ware, so the drips were drying before they had chance to run off the edges.  It's very easy to forget to take into account one of the factors which affect how the glaze behaves..not very exciting, but I'll list them here mainly for my own benefit, so I don't forget next time:

1. The thickness/density of the glaze slop. For years I tried to judge this by eye but it's much less risky to measure it with a hydrometer (or weigh the glaze exactly, relative to an equal volume of water).

2. The temperature of the bisque firing.

3. The thickness of the pot walls. The heavier the pot, the more glaze it will suck up. Glazing the inside of a thin pot will then affect the amount of glaze the outside surface can absorb. One may need to wait a day between glazing the inside and outside of a very thin-walled vessel.

4. The amount of time the pot is dipped for. I usually dip my pots for between 3 and 8 seconds, depending on what thickness I'm aiming for.

5. If gums like CMC have been added to aid suspension or glaze hardness, they may also affect speed of glaze drying and how much glaze is absorbed by the clay.

Anyway, by early evening today, I'd managed to glaze a number of pieces reasonably well..and the proof of the pudding will be in the firing tomorrow!

Never give up, the best is yet to come! 

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