Here's a question for potters and/or chemists and probably a lot of other people too.
Pottery plaster is mixed with water at a 10 to 7 ratio. That is e.g. a kilogram of dry plaster wants 700 grams of water. Similarly a basic recipe for refractory molds for glass casting is half dry pottery plaster and half silica flour with a dry ingredient to water ratio of 3 to 2.
The problem lies in predicting the volume of liquid plaster produced from a given amount of water and dry plaster. There's a lot of miscibility going on and it's not even close to the sum of the volumes. Andrew Martin* has a recipe that gives one quart of wet plaster and using the ratio it's easy enough to scale it up or down as needed. But there's no such recipe for the glass casting mix, and anyway using a recipe bothers me to some extent. What I'm looking for is a direct way from "I need x milliliters of wet mix so I should use y milliliters of water" and then use the ratio to calculate the amount of powder.
So what occurs to me is this: barring technical and irrelevant details like elevation about sea level and air temperature the specific gravity (= weight in grams of one CC) of each of these two mixes ought to be constant. Suppose way pottery plaster has a specific gravity d. Then if I need e.g. v cubic centimeters of mix I know I need vd grams of mix. If w is the amount of water I need and p is the amount of plaster, both in grams, then we should have:
w+p=vd
7p=10w
So 0.7p=w and therefore p=vd/1.7 and w=0.7vd/1.7.We can do something analogous with the plaster/silica recipe. A formula like this would be much easier to work with since d is constant so I could get the amounts directly with just two calculations.
My question is: will this work? I'm worried a little about the assumption that the weight of the wet mix is the sum of the weights of the ingredients. There's some kind of chemical reaction happening with the water and the plaster and it wouldn't be all that strange if some weight was gained or released at that stage e.g. if some atoms are related or absorbed. The thought of mixing and measuring accurately enough is daunting so I thought I'd ask fedi!
*If you want to learn slipcasting Andrew Martin's book The Essential Guide to Mold Making & Slip Casting is fantastic.
#Ceramics #Pottery #Slipcasting #MoldMaking #Plaster
Adrian Riskin :anarchoheart2:
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2026-08-13 16:46:55 UTC
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"content": "Here's a question for potters and/or chemists and probably a lot of other people too.\n\nPottery plaster is mixed with water at a 10 to 7 ratio. That is e.g. a kilogram of dry plaster wants 700 grams of water. Similarly a basic recipe for refractory molds for glass casting is half dry pottery plaster and half silica flour with a dry ingredient to water ratio of 3 to 2.\n\nThe problem lies in predicting the volume of liquid plaster produced from a given amount of water and dry plaster. There's a lot of miscibility going on and it's not even close to the sum of the volumes. Andrew Martin* has a recipe that gives one quart of wet plaster and using the ratio it's easy enough to scale it up or down as needed. But there's no such recipe for the glass casting mix, and anyway using a recipe bothers me to some extent. What I'm looking for is a direct way from \"I need x milliliters of wet mix so I should use y milliliters of water\" and then use the ratio to calculate the amount of powder.\n\nSo what occurs to me is this: barring technical and irrelevant details like elevation about sea level and air temperature the specific gravity (= weight in grams of one CC) of each of these two mixes ought to be constant. Suppose way pottery plaster has a specific gravity d. Then if I need e.g. v cubic centimeters of mix I know I need vd grams of mix. If w is the amount of water I need and p is the amount of plaster, both in grams, then we should have:\n\nw+p=vd\n7p=10w\n\nSo 0.7p=w and therefore p=vd/1.7 and w=0.7vd/1.7.We can do something analogous with the plaster/silica recipe. A formula like this would be much easier to work with since d is constant so I could get the amounts directly with just two calculations.\n\nMy question is: will this work? I'm worried a little about the assumption that the weight of the wet mix is the sum of the weights of the ingredients. There's some kind of chemical reaction happening with the water and the plaster and it wouldn't be all that strange if some weight was gained or released at that stage e.g. if some atoms are related or absorbed. The thought of mixing and measuring accurately enough is daunting so I thought I'd ask fedi!\n\n*If you want to learn slipcasting Andrew Martin's book The Essential Guide to Mold Making \u0026 Slip Casting is fantastic. \n\n#Ceramics #Pottery #Slipcasting #MoldMaking #Plaster",
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