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Which Type Of Plaster?

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Greetings all,


I have been making mugs with tiki faces carved in them.  I have made molds that have been working so I can reproduce them fairly quickly.  The problem is that the molds chip very easily, leaving holes in the mold which then create unwanted that then have to be smoothed out after the pouring and drying. 


My question is:  Is one plaster better or more durable than the others?  I have been using pottery plaster #1.  However, at my ceramics supply company that I order from I see that they have a number of different types of plaster:


Plaster, Hydrocal White with 4500psi compression strength

Ceramical Gypsum Cement with 6500psi compressions trength

Puritan Pottery Plaster with 2400psi Compression Strength

Pottery plaster #1 has a 2000psi compression strength. 


I'm going to go out on a limb here and guess that the higher the compression strenght, the harder the plaster.  If someone could confrim this, that would be great


Second question:  Who has used a "harder" plaster for mold  making and what are the disadvantages or advantages you have found?


Still kind of new to the mold making and production process and trying to avoid the years of trial and error, so any information would help a bunch.





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Hi Brian, I've only ever used #1, except for making ram press molds. I can say that the way you mix your plaster will effect the "hardness" of your plaster. With harder plaster, you're going to be dealing with different rates of absorption, and different saturation points (as far as how much moisture the mold can take on). This will effect how well your molds cast. Maybe someone else can say more....

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One of the most important ratios you need to understand is water:plaster ratio. It will make huge difference how your final mold behaves.

Same plaster can give you very different results. Last time we made molds we used about stetson hat more than a 1/4 of a metric ton of plaster. ;)

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It took me many failed molds to get comfortable with mixing plaster. I found that it was more in the mixing as long as the water is saturated with plaster. The longer I mixed made it better and stronger. I am still no pro, but my advice is to work with the cheapest plaster until you understand the ratios and mixing process to get the best from the plaster then buy the harder more expensive ones.

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Smaller amounts are more senitive to weighing mistakes-mixing errors 

(Agian, trying to avoid the years of trial and error.)This is how we learn in this field and as the years go by you will learn that you never get it all down as errors are the other name for ceramics

Read up on how to mix  for best results

I googled it for you 

Heres some good info on small batches




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  • 3 weeks later...


Are you weighing your materials beforehand? Weak plaster is usually caused by high water to plaster ratio. I used USG #1 Pottery for many years and did not experience much chipping.


I don't weigh my materials everytime I pour but I do when I get a new batch of plaster. (So I can gauge how old the plaster is.)


Pottery Plaster and GP K-60 are the best for slip casting. GP Densite and USG Hydrocal are usually best for ram pressing. They can be used for casting but your set up time will be much longer.


Good Luck


Jeff Longtin


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