VACUUM! Degassing plaster to remove bubbles? Maybe not

For such large objects, the results are great! Thank you for the explanations

Kent, you're always thinking, and it makes me happy to follow along. Thank you for making videos, you are teaching us all as you experiment and learn.

Today I am a ceramicist, but I used to work as a dental technician in a plastering plant. There are vacuum mixers for plaster, but they are certainly very expensive equipment and few prosthetic studios had them. It is important to sprinkle the plaster over all the water in smaller portions. Until you stir it, it doesn't start to set. it can stand there untouched for a few minutes. It then sinks and becomes easier to mix. When he gets involved, the bonding begins. I did it by hand, but with much smaller amounts of plaster... and I always poured it into the plaster using a prosthetic vibrator. It vibrates completely differently than yours. It has adjustable vibration intensity and a removable rubber on the counter for cleaning set plaster. I also always sprayed the model with an agent that removes surface tension, but this is probably mainly useful for complex forms with small details (like a tooth model). Thank you so much for sharing all your noledge. I hope to use it soon also❤

My dad is a jeweler and he has a pretty involved process for dealing with bubbles in investment for lost wax casting. I don’t know if any of his process can transfer over to your process but I’ll share it just in case. He doesn’t bother with vacuuming until after he has completed the mixing process. Directly after mixing he goes straight into the vacuum chamber. Then he pours the vacuumed mixture directly into the mold which in his case is a flask. He then puts the flask in the vacuum chamber and vacuums a second time. After that he puts the flask into a pressure chamber. The pressure chamber compresses the remaining bubbles to be as small as possible while the investment sets. I can’t remember what the timing of all this is but it’s very precise and fast

Suggest that you add plaster by sifting it into the water over a number of passes , this brings the plaster to the water without drawing air in as can happen when you dump larger drops of plaster, into the water , add the plaster slowly to the water , The plaster does not start to set at this stage and you can leave it stand like this for some time, the chemical reaction happens after mixing/movement after slaking. I think that applying the vacuum to the slaked mix is doing two things it is not allowing the slaking to happen and is essentially starting the mixing process as the air is rising in the mix and is most likely the reason that the mix is setting after a double agitation , applying a vacuum to the Slaked mix (moves and agitated the mix) and then stirring and adding the vacuum again essentially gives the plaster nearly 5 minutes of mixing and no time to slake. Traditionally this leads to a very early setting plaster. Maybe try sifting the plaster as you add it to the water , let the plaster slake for 2 minutes (untouched) , then mix and only then add to the Vacuum Chamber to degas before pouring..

I use a shoulder massager against the side of the pail for 30-45 seconds after mixing and then against the mold in a few spots around the bottom for a couple seconds after pouring.

Looking good. You could be drying the plaster faster since your converting the water to vapor at lower pressure. You need extra water in the vacuum chamber to prevent the plaster from drying out. I wonder if you can fit a hygrometer inside your vacuum chamber.

One thing that I've done to cut down on surface bubbles is to mix a very small amount of plaster and then either brush or dip coat on a thin layer onto mold pieces, let it set, brush or dip coat on another thin layer and perhaps another, then do the main pour after I've manually applied a few thin layers. In lost wax metal casting this is one of the ways surface bubbles are avoided. With your specific mold you could probably just assemble the mold, pour a little bit in, and then rotate the mold in your hands until it has flowed over the whole surface and keep turning it until it cures into a very thin layer, then do the main pour.

You can also use a paint defoamer for water based application. It will work wonders. Normal dosage is .25% but they can be used even more upto 1% of total mix. Add defoamer in two parts. Measure the quanity of defoamer then mix half of it in water that is going to be used with plaster using mixer and then add plaster. Add remaining after mixing the plaster. Also don't use any surfactants or soap. They will create more foam. As for vaccum chamber, keep the mix less viscous. Plaster tend to get thick when it starts to set, so add a retarder so that plaster will remain fluidic for more time and can be easily degassed in vaccum chamber. You can also add litte extar water to make mix more fluidic. One more trick is add little bit of retartder to mix and keep the mix for 5-10 minutes. All the foam will float to top . Then spray IPA / Alcohol/ Vodka on the foam. This will destroy all the foam.

I'd like to see what kind of deformation you would actually with degassing the mold. You could try a small piece without plaster. For my castings, I use a massager to vibrate the form. I can tell that the liquefaction really makes a difference even when the plaster is getting too thick to pour. Vibrating a container with a small amount of plaster left in it will make the plaster flow waaay better. I also spend 4-5 minutes vibrating different spots and rotating the mold to try to help the bubbles out. Even after several minutes, bubbles still come out. For your mold, since the inside is open, you could probably hit that particular part with some localized vibrations to have those bubbles release...

ive zero experience with plaster/ceramics and i only watch your channel for fun. but i do have some experience with composites. what we do to mitigate bubbles/pinholes is a layer before the actual lamination. i wonder how well a brushed layer of plaster on the male part of your mold would work, with the casting coming after this first layer is cured.

It's fun to think of ways one might perform the stirring in a vaccuum. I mean, how can you introduce air while stirring if there is no air, right? beside magnetic couplers, I wondered if you could perhaps detach your pot+lid from the hose (since you clearly have a valve) and SPIN the entire pot so vigeriously that the slurry mixes itself... Like a giant protein shake shaker :)

hello.your videos are really inspiring. Do you have the stl file of the tool you attached to the tip of this drill? I want to print and use. And how do you adjust the ratio of water to plaster? Do you have a recipe for grammage?

Will your vacuum chamber fit on your shaker? Try pouring like normal first then vacuum and shake the entire mold.

This may be of no use to you, but my grandfather was a plasterer by trade and he taught me to use a small amount of distilled white vinegar to delay the setting of plaster long enough to sculpt intricate Victorian moldings for architecture. This might be of some use in delaying the set of your pottery plaster long enough to vibrate the bubbles out with something like passing an electric sander over the outside surfaces of the mold or on that table doohickey you got from the lab.

Would pulsing your drill, rather than going full pelt, stop the dreaded vortices?

What you are doing in the vacuum is boiling the water. look at charts online for the boiling point @ temp @ pressure of your chamber. stop vacuum prior to the boiling point. Some ideas for you; use cold water. Put the mold into the vacuum chamber pour ice water around the mold, fill the mold with your gypsum mix and quickly pull a max vacuum and leave it there for 30 minutes. I am not too optimistic on the outcome; but would like to see you try it. I think a pressure pot would be more appropriate for your application.

what about changing the pouring method? a funnel and a hose to cause less turbulence as you’re pouring the plaster? I’ve seen resin people use a brush to remove bubbles on the surface. Maybe that’ll translate to plaster

What if you use one of those magnetic lab string gadgets to stir it while it's under vacuum?

What vacuum are you using? Have a link?