As you can see from my studio photos, the last few weeks I’ve been concentrating on making one piece moulds. I’ve been making traditional block moulds, because the objects I’ve been casting (tiles) are small and angular.

As part of this process, I began to wonder about how to calculate the amount of plaster I needed to use for each particular mould. I want to conserve my resources, to be economical and to avoid being needlessly wasteful of materials.

I had a formula that my friend Beth gave me last year, which involved working the whole thing out in inches, pounds and quarts, which, although it works well, needed to match the wiring in my brain (and my rulers, scales and measuring jugs), which unfortunately is strictly based in the metric system.

I searched the books I had at the studio and all I could find was that 1L of water takes up a volume of 1000cm cubed. (I’ll write that as cm3 - Not sure how to make superscript in this program yet). I proceeded to work out, mostly by trial and error, a formula which would let me calculate the exact weight in plaster (in grams) I needed to use to fill a cottle of particular volume. I know this is probably available in all sorts of texts, and perhaps is even common knowledge, but since I’ve worked it out myself I’m particularly proud of it and wanted to share it with anyone interested. Here it is:

First, work out the volume of your cottle (Make sure this is the INSIDE volume of the cottle) I’ll use one of mine as an example:

Volume = Width x Length x Height (in centimeters)

= 19.3 cm x 19.4 cm x 4 cm

= 1,497.68 cm3

= approximately 1,500 cm3

Now, we know that 1L of liquid takes up a volume of 1,000 cm3, so the volume of my cottle is 1,500 ml or 1.5 L.

Next, work out how much water and plaster together will make up that volume. (Here the number 1.5 is what I worked out by trial and error).

This is how to work out how much water is needed:

X ml of water x 1.5 = total volume of cottle (ml)

X ml of water = total volume of cottle (ml) / 1.5

= 1500 ml / 1.5

= 1000 ml

= 1 L

So the total amount of water I need to use to make this mould is 1L

Now work out how much plaster is needed:

I have a formula from a workshop by Trudy Golley, which I use to work out my water to plaster ratio for any mould. It is 1 part water to 1.3 parts plaster by weight. Using this formula means that all my moulds have the same consistency and same absorbency. This is particularly useful when making moulds with multiple parts. A key part of this is knowing that 1L of water weighs 1kg (so 1ml of water weighs 1g).

Amount of plaster in grams = volume of water in mls x 1.3

= 1000ml x 1.3

= 1300g

So, the total weight of plaster needed is 1,300g or 1.3kg.

If I add 1.3 kg of plaster to 1L of water, I fill a cottle of 1,500 cm3 volume exactly. Try it and see; it works!

I have since found a similar formula in Anton Reijnders’ book. He seems to however work backwards based on the weight of his water/plaster mixture and uses a water to plaster ratio of 1.5.

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This entry was posted on Sunday, August 26th, 2007 at 2:38 pm and is filed under Ceramics, Glasgow Ceramics Studio, MPhil, Scotland. You can follow any responses to this entry through the RSS 2.0 feed. Responses are currently closed, but you can trackback from your own site.