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Do plaster molds have a limited number of uses?

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Hello Everyone!

About a year ago I created my very first plaster mold.  I LOVE IT!  I have been very happily slip casting again and again in my one single mold.   I figured that over time the mold would wear out, but I thought that wear would come in the form of chipping or accidently dropping it.  Instead, it doesn't appear to be absorbing much water anymore.  

When I first made the mold, it was fast!  12 minutes to get the wall thickness I wanted.  Then it was 15, then 20, and now after leaving my slip in the mold for over 30 minutes the wall thickness remains very thin.   I just doesn't seem to absorb water anymore. 

Does the plaster eventually lose it's absorbing abilities?  If so, how many casts can one expect from a mold?  Or do I simply need to really let it dry between casts?  If so, has anyone found a good technique for that?  Putting it outside in the sun perhaps?  Using it only once every two weeks or so?  Is there a maintenance or schedule to using a mold that I am just not aware of?  

Thank you for any and all guidance.  


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I'm not a mold maker, so hopefully someone with more knowledge than me chimes in, but I'm thinking there are two possibilities:

1. You need to let the mold sit out for a few days, even a week, to totally dry out. It may just be saturated and isn't drying enough between uses.

2. The absorbency is decreasing due to the buildup of soluble materials in the plaster.

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Your one off mold needs time to dry-blow air over it with a fan on . That will speed dry it.

I had a slip business at one time and we poured about 100 of 3 piece molds a week-we rotated molds in and out of the system-always kept the fan on them when not using them. I live is a damp climate. 

Over the years they wear out-at the joint line (that crack gets larger) and they will get surface pits on the plaster. You can always dry them out as noted in above post 

(The absorbency is decreasing due to the buildup of soluble materials in the plaster.)  This takes many many pours -we made new molds about yearly but we where casting thousands of pieces

 you use will make a difference -thingslike soda ash and sodium slicate will shorten the life if thats in your slip-we only used Darvon 7 as an additive -those other ingredients will shorten your plaster mold life.

Edited by Mark C.
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To answer your question directly, Yes, molds have a limited number of uses. As Neil and Mark have stated, the clay you use, and how you maintain it, has the biggest affect on mold deterioration.

The other aspect of mold deterioration is subjective, what you may find acceptable, and still usable, may be different then I, or the next person. One customer commissioned me to make 10 molds to replace her existing 10 molds. She told me they lasted thru 150 pours. I was astounded. She then explained that the pieces required a lot of cleanup. While the cleanup was time consuming she accepted the delay as she was unable to find a mold maker to replace them. She was overjoyed, when she found me, and overjoyed that she could finely replace her aging molds

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