Plaster bubbles

[purplecat]

How do I prevent my plaster from having bubbles when I cast a piece? Does the type of plaster matter? I'm doing test casts in a cheap plaster of paris and it's bubbling horribly...will I have the same problems when I do the actual production castings in Drystone? What tools are best for mixing plasters? Thanks!

[rhendriks]

You need to use the right proportion between water and dry plaster,otherwise the plaster becomes to thick and you has to stir it.
I pour in 2 layers.In the first layer(which covers the model thin) I remove the left bubbles with the back of a plastic spoon
touching the surface of the liquid plaster gentle,the movement removes underneath airpockets.

[HappySculpting]

Hi Purplecat,

Coating the surface to be molded with a good quality mold rinse has worked wonders for me. I've heard that the mold rinse has talc in it and this seems to attract the plaster to the surface and creates a no bubble surface. (Definitely helps to tap the plaster bucket on the ground very vigorously to work the major bubbles to the surface before pouring into the mold. ) Plaster No. 1 is a good standard for ceramic mold making.

~Tamara

[Merlion]

When the plaster is well mixed with water, knock or tap on the container wall to vibrate and speed up the rising to the surface of bubbles.

After this mixed plaster is poured into the mold, shake and tap the mold so that any bubbles sticking to the mold surface have a chance to be vibrated loose and rise to the surface.

Sometimes the shape of the mold traps air. To avoid the air being trapped, before the plaster is poured up to the top level tilt the mold around to allow trapped air bubble to rise up and get off.

When you inspect a plaster cast after the mold is removed, look for any holes on the surface left by these bubbles. If there are, the holes can still be filled up with plaster.

[racine]

also see 'need mould making material'.. current topic in sculpture focus above..oddly. if u mix the right stuff Annes way then its cool. for further info i have added to that thread. when its mixed right the bubbles rise, no effort.

[purplecat]

Thanks! I'll try out these suggestions and let you know how it goes!

[dondougan]

Knocking the the side of the mold or flask works for the bigger bubbles, but for smaller bubbles a vibrating motion works more efficiently. Small platform-type vibrators such as are made for jewelry casting will work to release the bubbles when casting small molds, but if you have any tools in the workshop such as a jitterbug-type vibrating sander or a pneumatic carving hammer you can hold them (while running) against the side or bottom of your mold or flask for the same effect.

Another thing that helps with bubbles is to mix the plaster gently with your hand (by squeezing the plaster under the surface) rather than stirring with paddle or whisk. That way you can prevent the bubbles from getting mixed-in in the first place. And of course letting the plastermix stand and slake (don't start stirring right away, but allow the water and plaster to settle completely undisturbed for perhaps five minutes) will prevent both lumps and bubbles.
When you go to pour the plaster in the mold or flask, hold your mixing container up fairly-high above the molding/casting container and pour with as thin a stream as possible, pouring at a steady even rate into a single point in the mold rather than just dumping it in or moving the impact point around -- the thin steady stream will help remove existing bubbles from the mix, while not moving the impact point will help prevent new air bubbles from forming on the surface of the mold/model.

Don

[Landseer]

I never use mold release, nor tapping or vibrators, I don't even have to wet the rubber before pouring.

I simply make sure not to mix more air INTO the hydrocal mix when mixing, then when filling the mold I fill it half or 3/4 the way up and dependng on the size/depth of the mold I just don a rubber dish washing glove if the mold is not deep, no glove if it is as the stuff will wind up inside the glove- and just work my fingers over every bit of the surfaces inside.

I use a jiffy mixer and electric drill, you MUST mix USG's products like hydrostone, hydrocal, duracal etc etc mechanically and for a specified time- I mix for 90 seconds on high speed, or you don't get the strength.
The jiffy mixer must be kept at the bottom of the pail which should always be full otherwise the turbulance will make the slurry whirlpool in the bucket and suck in a huge amount of air.

It only takes a few seconds and your fingers rubbing around inside the rubber dislodges all bubbles trapped in the details, and your hand swirling around in there creates turbulance that further dislodges more.

My casts come out totally bubble free every time regardless of size, design, undercuts etc.
The same applies for concrete, though a glove is REQUIRED, but concrete being much thicker is harder to work, but complex casts come out good.