Leather hard to glaze fired weight difference

[Tom Humphries]

Hi all. I've made my first set of dinner plates and have just finished trimming them. I think they might be a bit heavy. I was wondering if anyone knows what percentage difference there is in weight between leather hard and final glaze fired weight? I know there are tonnes of variables like how much it's dried, glaze thickness, clay type etc. But can anyone give a rough estimate?

Thanks,
Tom

[Norm Stuart]

Hi Tom - 100 grams of wet clay should weigh about 52 to 57 grams after it's bisqued or high-fired without a glaze.

Prepared clay is usually 1/3 water by weight. You'll have to weigh a piece of wet clay, then weigh it again to learn how much water weight is lost getting to leather hard.

When you bisque-fire clay it loses another 12.5% of weight as chemical water is released by the kaolin to metakaolin conversion and you'll lose any organics in the clay - so from bone-dry to bisqued you can count on a loss of 13% to 21%.

100 grams of Wet Clay - 33% = 66 grams - 13% bisque = 57 grams or as little as 66 grams - 21% =  52 grams

The weight added by glaze varies enormously.

Thin ceramic pieces, which don't weigh much, can easily double in weight after they are heavily glazed and re-fired. A piece bisqued to a higher temperature like cone 03 or 04 will absorb less glaze than one bisqued to cone 06. More of a stiff glaze can remain on a ceramic without running off, so this can load up glaze weight, as can a thick beautiful pool of glaze inside a bowl.

Glaze adds very little weight to bone china or fine porcelains because they are first completely densified and made non-porous at Cone 02 to Cone 12 depending on the vitrification temperature of that prepared body. A very thin coat of wet glaze, or dry glaze applied with an electrostatic charge, is added after firing then refired to a much lower cone, often cone 2. Any gilding or colored enamels are then applied and the china is refired again to cone 016 or so. The total glaze weight will add only 5% to the total weight.

In contast, for fun I made a large and very thick fountain (about 14 to 18 mm thick) with a very porous clay with a lot of grog, Laguna Big Pot. After being bisqued this porous clay absorbed so much glaze to become water-proof that the glazed and fired fountain weighed as much as the original wet clay used to build it.

This fountain weighed as much as the original wet clay (about 41 kilos) because the porous bisqued clay took on 50% of its weight in glazes. It sits in our studio as a "white elephant".

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Top view showing how thick the piece is and the drip-irrigation plumbing of the fountain.

[Tom Humphries]

Thanks Norm that is all helpful information. Lots of variables! I decided to go with my gut feeling that they were a bit heavy. I trimmed a bit more as I had a rather chunky footring. I've recorded the weights of the four plates (average 850g) at leather hard. I will post the fired glazed weights for anyone who is interested.

[Tom Humphries]

Fired glazed weights average at 760 grams

[Norm Stuart]

Interesting. That 10% loss of weight would probably be quite different with a different clay and glaze.