I have just finished making over 200# of plaster molds for our local Art Center and have come up with a few questions. This is not my first time to make , but with the quantity I made, I seemed to have a variety of different ‘problems’. I actually only ‘lost’ about 5 pieces of 50, so I feel quite successful as I do not work with plaster that often. With your help, perhaps what I perceived as ‘problems’ will not repeat in future mold attempts.
For my molds I used new pottery plaster.
I weighed out my water and pottery plaster, as directed with my molds, to the ratio of.7 .
Plaster was sprinkled over the water and then left to slake for 2 minutes. I then mixed gently from the bottom, getting rid of all lumps. Then I let sit a couple minutes more, gently mixed, and poured it gently into silicone sprayed molds. (Most of the time the plaster settled/slaked under the water but several times not all of the plaster settled so I had extra mixing – I did not notice any difference in my water temps or time – any ideas why it did not settle – plaster set up quickly).
I learned that water should be cool rather than warm or it sets too quickly.
Several times, altho the plaster was mixed well, as it set in the molds, a thin layer of water formed on top – what did I do wrong? (I just laid a paper towel on the plaster to soak up the water.)
Also, twice the plaster did not even get warm - why? But it set up and appears OK.
I thumped my molds on the floor after filling to get rid of air bubbles but occasionally I had a few, very small, bubbles show up. They don’t look large enough to cause surface problems in my clay – we’ll see.
I have been told to wait to fill the molds until you can draw a finger across the mixed plaster leaving a slight indentation in the mixed plaster surface – is that right? At times it then seemed to set very quickly once I started to pour.
I did learn that different brands of silicone release differently.
Molds have dried nicely by placing them in the area of our wood burner – can do a lot more at a time rather than in an oven! However they do take about 4 days to dry.
Thanks for your comments! They are always appreciated! I learn so much from this forum!!