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Making Plaster Molds


Diz

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Hi All,

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!!

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Cold water is what to use-you got that. as warm water affects the set time.

I  always after mixing with a power mixer drop the bucket on a firm floor a few times to make any air bubbles rise-this helps before pouring begins.

The water on top is natural and will happen now and then-seen it many times.

As long as you have the portions right you will always fine a few variables creep in.

Tapping  the molds will help float up those air bubbles as you noticed

I never wait after my mixing time is up-as the plaster will start to set and you can have other issues as it does while you pour.The air bubbles rise best in thinner just poured plaster vs firm setting up plaster which is another reason not to wait.

Back when I had a mold business I learned learned a lot after pouring hundreds of molds-sounds like you are well on the way.

I have never used silicone as a release I always use mold soap . Does the silicone affect the plasters ability to absorb water after they are dry??

Mark

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I agree with Mark. You are waiting too long to pour your plaster. once you have it mixed thoroughly, pour.

I use Murphy's Oil Soap as a mold release. Can be purchased at the grocery store.

It is possible to burn your molds so that they become brittle and not absorb slip. Do not set them too close to your wood stove. let them dry slowly.

TJR.

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Hopefully the silicone will not cause problems for me.  That is what I was told to use by Mark at Continental Clay.  I sprayed it on the molds and then wiped them lightly with a soft cloth to eliminate any bubbles that formed in the silicone spray.

 

Our wood burner is actually a wood burning furnace so the heat is very gentle so they take about 3-4 days to dry - no visible problems with drying so far. 

 

I will drop the bucket on the floor tho - that sounds easier and more efficient than thumping each mold.

 

Thanks for the info - it's nice to know that I had no major problems with my 'technique'.

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I don't make a lot of molds, so I just use thinned liquid hand soap from the bathroom and it works well. You have to be careful not to make bubbles, though. I agree that you shouldn't wait so long to pour the plaster. I sift the plaster into the water, let it sit for 3-5 minutes, then mix with a stick blender (small batches) or drill mixer (big batches), then pour right away. As mentioned above, thump the mold a bunch right after pouring to get out air bubbles. Don't let the molds get too hot during drying, or they can degrade. I just made a mold  recently that took about 10 days to completely dry, because my studio isn't all that warm. 3-4 days would be great.

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Make sure you sieve the plaster, it takes a while but it makes all the difference. You shouldn't be breaking down lumps in the mixing process. If you sieve and leave it to soak for a few minutes then it should be easy to mix. I also mix for 2-3 minutes by hand until I feel it thicken and the surface is a little glossier than when I started. This is what I judge to be a good pouring time.

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