Mixing Hydrocal with #1 Plaster for Added Strength

[colekeller]

Hey all, 

I'm working with a new model and plan to make a plaster mold to cast silicone replicas of my original model for easy mold replication. I've had issues in the past casting pottery plaster around plastic models (rigidity of the plastic causing excess chipping on edges). I have some hydrocal but not enough to make the entire mold. I was wondering if you thought it would be possible to mix pottery plaster and hydrocal together (say in a 1:1 ratio) to increase strength of the molds? Any help or information would be greatly appreciated - I don't have much hydrocal and I live quite far from my local pottery store so I'm hoping I don't need to run a test first for the sake of material rationing.

Thanks so much!

Cole

[oldlady]

i believe that is exactly what Joan Gardiner does when she makes molds for her fabulous tiles.   look up Unison Pottery and Tile in Virginia for contact info.

[colekeller]

@oldlady Thanks for the info! Her work is absolutely stunning!!! 

Cole

[ZaneConcept]

Hi Cole,

Also looking to strengthen Hydrocal and curious to know if you had any luck?

thanks

zane

[oldlady]

the same advice cole used is still good.

[Jeff Longtin]

In case folks don't know: the plaster/cement products develop strength the longer that you mix them. If you pour them when they are water thin they will tend to be weak/soft.  If you mix them until they become heavy cream-like they will be much stronger. (Also using slightly cold water helps.)

If I'm pouring a basic shape, with no undercuts or fine detail, I will continue mixing until I can see the plaster starting to thicken/set. Once I pour it into the mold box I use a spatula to gently vibrate it into place. (By vibrating the plaster gravity helps it self-level.) 

If I wait until the last minute, to pour, the plaster will set up instantly and be rock hard.

Also: if you pour your plaster and you see a slight layer of water form on the surface that's an indication that you poured too early.  Ideally it should harden before that happens. 

 

 

[PeterH]

5 hours ago, Jeff Longtin said:

In case folks don't know: the plaster/cement products develop strength the longer that you mix them. If you pour them when they are water thin they will tend to be weak/soft.  If you mix them until they become heavy cream-like they will be much stronger. (Also using slightly cold water helps.)

When mixing modest quantities I was taught to stir gently by hand, fingers apart and not breaking the surface. The flow between your fingers  gives a feel of how the hardening/homogenising process is going.

PS But do the plaster into water bit first, as at the start of
https://www.youtube.com/watch?v=PYvAIeSyhgw

PPS Of course take your hands out well before the plaster sets!
Art class girl loses eight fingers after putting hands in plaster of Paris
https://www.mirror.co.uk/news/uk-news/art-class-girl-loses-eight-424409

[Jeff Longtin]

Peter,

If that method works for you then by all means stick with it.

I did not have a teacher. I am self-taught. My information came to me from company info sheets. Those said that its best to agitate the mix as much as possible. It has worked for me for over 30 years.

The method you describe sounds like its intended to avoid air bubbles as much as possible. That's fine but there are ways to remove air bubbles easily and effectively.

First method to remove air bubbles is to hold a spatula in the plaster mix and gently create a wave. This causes air bubbles to rise to the surface. Then I either pop the bubbles with said spatula or I spray a light mixture of water and rubbing alcohol. The third method is to swirl the bucket. This works well for smaller quantities.  As you swirl, the bubbles come to the surface. As you continue to swirl, the bubbles are broken.

Another method to remove air bubbles, from plaster pours, is to stick your fingers into the plaster AFTER you poured it into the mold box. (A soft bristle brush also works well.) Brush the molded surface with your fingers, or the brush, and actually move the bubbles from the model surface. (A tad messy but very effective.)