Ryan Clyde-Rich on sun 8 jul 01
When I make glazes, especially tests, I want to be able to control the
amount of water that I need to add to the glaze to get density around 1.4.
If I have a glaze test that adds up to 100g without water, how do I
calculate the amount of water that I need to add so I can get a specific
gravity of 1.4. If anybody has an answer to this one I would really
appreciate some help.
Randy and Cheryl Weisz on sun 8 jul 01
I also have wrestled with this. In the back of Harry Fraser's book:
Glazes for the Craft Potter
he references Brogniart's formula. This equation estimates the amount of
dry glaze material in a given volume of mixed up wet glaze assuming you know
the volume of wet glaze that you have, and the weight of that volume of wet
glaze. It also makes the assumption that the specific gravity of the DRY
glaze materials is 2.6.
You can re-arrange this equation to solve your problem. The result is:
Total Wet Glaze Weight = Dry Material Weight x 100 / 46.4.
This assumes you want a final specific gravity of 1.4 (the result would be
different for a desired specific gravity other than 1.4), that you know the
Dry Material Weight you have mixed up (in grams), that the specific gravity
of the dry materials is about 2.6.
Example: You have mixed up a test batch of glaze. The base glaze total is
100 g, and you added 8 more grams of coloring oxides for a total dry
material weight of 108 g. You want to add enough water to get a specific
gravity of 1.4. Then you would add enough water to the dry material to
reach a total Wet Glaze Weight of:
Total Wet Glaze Weight = 108 g x 100 / 46.4 = 232.8 g
I checked this against four test glaze batches I made up the other day that
I knew exactly how much water and dry materials I added to get a known
specific gravity. The results using the equation were very close to the
actual ones I got in the studio. At best, this is just an approximation.
I'm not sure that all materials really have a specific gravity of 2.6.
Also, I have some glazes that seem to need to be mixed up to a specific
gravity much higher that 1.4. So I would never just use this equation to
figure out how much water to add and then add it all at once.
Hope this helps. Please let me know how this works out for you.
> From: Ryan Clyde-Rich
> Reply-To: Ceramic Arts Discussion List
> Date: Sun, 8 Jul 2001 01:51:41 -0400
> To: CLAYART@LSV.CERAMICS.ORG
> Subject: Glaze Specific Gravity
> When I make glazes, especially tests, I want to be able to control the
> amount of water that I need to add to the glaze to get density around 1.4.
> If I have a glaze test that adds up to 100g without water, how do I
> calculate the amount of water that I need to add so I can get a specific
> gravity of 1.4. If anybody has an answer to this one I would really
> appreciate some help.
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Dave Finkelnburg on sun 8 jul 01
See Brogniart's formula, it relates raw material specific gravity to
slurry density. Was developed for slip, works for glaze, too.
You can easily work this out from first principles, also. It's simple
algebra. You want a finished specific gravity of 1.4? Assume your raw
materials (clay, silica, feldspar for example) are about 2.65 specific
gravity. The specific gravity of water, by definition, is 1. The equation
to solve then is:
100 grams X 2.65 + x grams H2O x 1 = 1.4 (100 + x)
Solve for x.
Brogniart's formula, by the way, is just a solution of this equation,
using a specific gravity of the raw materials which he found were typical
I hope this helps.
Dave Finkelnburg in Idaho