VACUUM! Degassing plaster to remove bubbles? Maybe not

In the ongoing quest to eliminate bubbles from plaster slip casting molds, I decided to go the overkill route and try using a vacuum chamber. Unfortunately the jury is still out on this one. Clearly there is a lot of air in plaster (even when it is just slaking before mixing). However the time to mix is fighting against the time to degas so the plaster is setting before all of the bubbles are removed. More investigation required!
For those following along with my mold making software - stay tuned! I'm making progress. If you want to sign up for the wait list use this link: shapecastmolds...

[@JuliaMostowska]

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❤

[@PotterybyKent]

Thanks for sharing all of the info!

[@mandark10110]

really solid information thankyou!

[@mandark10110]

i like how your using PLA for your form then TPU for the softer barrier. I use a technique of a 3d printed cavity mould with a structure inside where i pour liquefied monster clay into it , when I open it up i am able to carve a design into the "vessel" but it ensures continuity (shapewise) between designs, so i can standardize my mould boxes for plaster etc...your channel has been extremely helpful in streamlining those processes so thankyou!!

[@PotterybyKent]

Glad it has been helpful!

[@theabristlebroom4378]

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.

[@PotterybyKent]

I appreciate that!

[@jakehyer1817]

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

[@PotterybyKent]

Thanks! I hadn't thought about looking into jewelry making but I could see bubbles there being very problematic! I've seen the process of vacuum to pressure being used for other things as well (like resin). I've been worried about my 3D printed parts surviving pressure - but maybe I should check. Also, I think one of the keys here is that my pressure was too low so the water boiled. And related is that plaster does set pretty fast! So there's not a ton of time for the vacuum to do its trick.

[@jakehyer1817]

@@PotterybyKent that is a good point about the 3D print being affected by the pressure change. I didn’t think about the air pockets caused by the infill of the part. The only way to avoid that would be to have no infill and do solid wall prints which would take forever and a lot of PLA unless you only did like 2-3 layer thick walls. Anyway I really appreciate your videos. I can tell you have a very analytical approach which I appreciate. I definitely want to get into this kind of stuff with my 3D printers. Just gotta go build myself a kiln first. 😂

[@PotterybyKent]

@@jakehyer1817 Exactly and I'd rather not print things solid! However it might be worth trying with a normal print anyways. Good luck with the kiln! In the mean time you might check out kilnshare.com - no affiliation but it's a place people use to put their kilns to rent out (mine is there as well).

[@chaotic_goodness]

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.

[@PotterybyKent]

Yes, I think vibration can contribute to moving bubbles away from the surface (similar to the vibration tables). I am starting to wonder about which variables associated with vibration might move the most bubbles - there's timing, location of the vibration (is it the whole mold or localized), frequency, amplitude, direction, others?

[@peterflynn9586]

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..

[@PotterybyKent]

Yes I do think the agitation as the air is pulled out starts the mixing process early. And as others pointed out, the hard vacuum is boiling the water. Given that, it's hard to draw any other conclusions. I'm curious about the idea of sifting. Naively that would aerate the dry plaster. However I guess if it is applied slowly enough I guess each particle becomes hydrated when it contacts the water and maybe that reduces the air? I'd like to understand the mechanism there better. Likewise, I've heard that letting plaster sit submerged doesn't start the setting, but that also doesn't seem exactly right (as dry plaster in water does eventually set). Maybe there is a speed component? That is also something I'd like to more of the science about.

[@FlorinGN]

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

[@uploadforgraduation9]

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.

[@PotterybyKent]

Putting on a thin coat first and using a brush to get the bubbles out is an interesting idea, as is putting down a thin first coat. I wonder if the double pour might affect water absorption from the slip.

[@abhishekbisawa6883]

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.

[@PotterybyKent]

Thanks for all of the tips! This is great. I didn't know paint defoamer was a thing. And likewise I hadn't come across plaster retarder before! I'll need to look into those. Both seem like they could be useful here for sure.

[@ringles1201]

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.

[@PotterybyKent]

Thanks! I got a DM suggesting something similar, and it does seem plausible. I'm not sure there is a good way to measure water content of the air or (wet) plaster in the vacuum chamber. I could try adding more water, but I don't have a good way to test the impact that has on the resulting plaster. (However if I'm losing water, I have an unknown there too).

[@sealpiercing8476]

@@PotterybyKent OK so you can control the humidity in the vacuum chamber by having some loose water--not in the plaster mix, just sitting in the bottom of the chamber. That will prevent the residual gas in the chamber from getting any drier than the boiling water at the bottom. That should be reproducible.

[@mattiasfagerlund]

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...

[@PotterybyKent]

That is a really interesting idea about using the hollow inside of the mold. That might be fun to try! As far as degassing the mold, my guess is that the 3D print would delaminate. These are printed with some infill (not 100% solid) so the pressure inside the 3D print would rapidly become greater than the outside and that air would want to escape. It is possible it might just leak out (my earlier tests with water showed there are often still small holes for water to get through). That could be easy enough to test without plaster. The other failure I could imagine would be in the seams of the mold system. But these are just guesses and some empirical evidence is always good!

[@mattiasfagerlund]

@@PotterybyKent I understand your reservations regarding gas filled pockets inside your 3D part. One thought I've had, though I've yet to purchase a vacuum pump, is that you could drill a hole from the "dry" side of the mold into any and all pockets - leaving the clay contacting parts pristine. However, this would be very far from your goal of a simple to use system. Anyway, I'm really enjoying seeing what you come up with and what you try - thanks for that!

[@PotterybyKent]

It might be able to be modeled in. This is a consideration for resin prints already - they need to drain. I haven't used a resin printer in a loooong time so I'm not sure how they handle that these days.

[@mrbilky]

Try using a palm sander once your pour the mold on the outside of the mold

[@PotterybyKent]

Thanks. I have

[@athompson8976]

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.

[@PotterybyKent]

Thanks. Others have brought up the boiling issue as well here in the comments, and I was inclined to agree. However I was thinking of revisiting this recently as I recently went looking at the pressure/boiling graph for water. I didn't record the pressure, but I would have only hit the boiling point close to the end as this vacuum pump wasn't all that fast. So I might have gotten there, but now I'm not sure that was really what was going on.

[@8Bit_ch]

maybe i am stupid but it looks like your water starts boiling. would be counter productive turning water into gas if you wanna degas the mixture :D *edit: yea i think you went to hard on the vacuum. boiling temp at 20 mmHg is just 15°c so it wasn't air inside the mix it was dehumidifying it (maybe)

[@8Bit_ch]

as far as i know they use vacuum for silicone, but i think some use pressure for concrete and stuff so due to the water it might work on plaster. it puts pressure to the remaining air so if there are bubbles they are really small..but again i don't know for sure :)

[@PotterybyKent]

Hmm - the water boiling is an interesting thought. It would explain a few things. I'll need to think about that more. The trick is you really can't go slower since the plaster sets pretty quickly.

[@PotterybyKent]

I haven't seen much concrete casting. But for resin and silicone, best practice seems to be to do both. Vacuum to remove as much as possible, and the pressure to collapse the remaining ones.

[@8Bit_ch]

the compressor is already there, thats half way to a presure casting. keep going it is fun to watch somebody tinker around :) @@PotterybyKent

[@Volksaholic]

I don't have first hand experience with what you're doing but I'm intrigued. I was suspecting what others have said: too hard a vacuum was probably boiling off the water. I think I'd try a lower vacuum. I was also thinking something similar to the magnetic drive for a mixer, but I was thinking about driving an impeller with magnets beneath the pot. If the pot is stainless without too much iron I think it would work. Maybe let it slake without the vacuum, then try mixing with a slow to moderate speed impeller in the bottom and a moderate vacuum. Probably still a lot of work for little gain but one never knows until one tries. :) Another thought I had that I imagine has been done is to run a vibrator between the inner and outer mold walls after pouring the plaster. When doing concrete form work that is a regular part of the process for minimizing bubbles.

[@jst1man]

Well i gotta say I'm surprised you didn't do the mix and later put it in vacuum in the mold, which by the way you do for epoxy resin. This can be very helpful. Now with that said, try Epoxy Resin and plaster mixed together. Just remember you have to use mold release! Epoxy Resin will stick permanently to most things.

[@PotterybyKent]

From what I've seen with epoxy, for the final cure it will usually go under pressure. Since the molds are 3D printed there seems to be some risk of deformation while under vacuum (however I have yet to actually test that). After this video and the discussion here in the comments, I'm not sure a vacuum will work since it lowers the boiling point for water. For resin that is fine, but for plaster the water is very important and boiling it off is an issue.

[@talastrogg1]

@@PotterybyKent I degas plaster on a form in a vacuum chamber. Printed shells are okay when they have some infill, but if there's a casted object inside, it must have a hole or be full; otherwise, it can explode.

[@Daily_Bugler]

Vacuum mixer… Some releasing agent Dental supply companies sell it… Rub on some mixed plaster with a vibrating plate… Then a pressure vessel while it sets…

[@PotterybyKent]

How big are the dental versions?

[@Daily_Bugler]

@@PotterybyKent you can see the mixing bowls size… approx about a 2L soda bottle in diameter to about half a soda bottle length… Some mixers hold the bowl in place… so your hands free to mix up a larger fill in batch… Then id rub it on with light vibration… then back fill… whipmix is a long established company … Dental technology supplies… Maybe use molten wax to fill any voids? With dental tech tools …. Tricky to explain the tech search KZbin dental wax carving … using vacuum mix hand rub n pressure pot i cant see you getting many air blows to be honest

[@Daily_Bugler]

Whipmix says 866ml with one slightly larger… Although Ive never used them in pottery synthetic dental stone plasters can be very strong n dense but im not sure how absorbent they’d be for your needs… Worth a try though…

[@PotterybyKent]

@@Daily_Bugler Thanks. For smaller molds that might work, but these are regularly 1-2L of liquid plaster in one batch. As for dental plaster and density, pottery plaster is actually very porous which is a key property needed for slip casting

[@hanelyp1]

Pulling vacuum slowly on the slaking plaster might avoid the early mixing.

[@PotterybyKent]

Maybe - it actually is towards the end when it seems to really get agitated which is a couple minutes in to the rather short slaking period (3.5 minutes). Given the way the bubbles raise more or less all at once after awhile, I'm inclined to think it is less about speed of the pressure change and more about the absolute pressure where all of the sudden the bubbles are let go.

[@pedrobalduci7754]

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.

[@PotterybyKent]

Thanks for watching! I very much like stealing ideas from other disciplines and applying them somewhere else! And yes that might work. I've gotten some comments about using a brush once the mold is filled with plaster to dislodge any bubbles. A thin coat might serve the same effect. I guess the issue there is either doing it in 2 steps so you have the time (and therefore increasing the overall time to make the mold) or trying to do the brushing and pour before the plaster goes off.

[@kitchentroll5868]

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.

[@PotterybyKent]

Interesting. I don't think I've heard about changing the pH of the plaster. I do know that temperature impacts setting time (cooler gives more open time).

[@quissum6418]

I’ve had some success with using pressure pot instead of vacuum chamber, and it definitely takes a lot of bubbles out (using Hydrostone).

[@PotterybyKent]

Do you leave the plaster in the pressure pot while it sets? I worry about 3d prints deforming since they are printed with infill and therefore have air inside them.

[@quissum6418]

@@PotterybyKent I’m casting in silicone and urethane molds that were degassed in a vacuum chamber. Yes, I leave them for the whole time. I did a few initial tests, and there’s a marked difference between pressure pot and not. I don’t know anything about how 3d prints would react - you should take a bad one and try it!

[@quissum6418]

@@PotterybyKent PS there is a video out there of a guy who filmed what happens in a pressure pot, and he had some plastic toys, and I don’t think they were destroyed.

[@PotterybyKent]

Thanks for the details. Using a solid mold makes sense. FDM prints have infill and voids, so they (should?) expand under vacuum. They might not be destroyed but could deform enough to impact the plaster's shape. Having said that, resin prints have a hole in them to drain excess resin. I wonder if something similar can be done with FDM to allow the pressure to normalize. (If anyone knows how to do that, let me know!)

[@hannesaltenfelder4302]

I started with degassing the water first. Maybe it is because I used fresh tab water, but the amount of air(or chlorine?) that came out was enormous! I used my wife's toy for vibration and the best place to use it is the middle of the lid. At the walls it is much less effective. Now I'm waiting... will update soon. Update1: Wrong ratio🙄

[@PotterybyKent]

Interesting that your water alone was degassing! Not what I would have expected.

[@hannesaltenfelder4302]

It seems like the water was boiling at room temperature because of the vacuum. I thought about that, but my misunderstanding was, that it would get warmer when boiling. But no, it is actually "cold boiling". kzbin.infohr55yM6eAS0?si=v2l9kcJUbMIoGMvG

[@PotterybyKent]

Ah that does make sense. I think the same thing was happening to me. I want to try again controlling for pressure so that doesn't happen.

[@LMShura]

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

[@PotterybyKent]

I think the brush would likely help. Unfortunately for my current molds it would be extremely hard to get in there with a brush. As far as a tube, that would probably just move any bubbles in the plaster (although might help not introduce them). For thinner material, I've seen pouring in a long thin stream - but I'm not sure how well that would work since plaster is so viscous.

[@Tydwhitey]

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 :)

[@PotterybyKent]

That would be a machine! Unfortunately I'm pretty sure the vacuum is causing the water to boil off at room temperature and that is messing with the plaster. So I'm not sure there's a path forward here. Maybe a partial vacuum? But in that case there's still air around.

[@dilian245]

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

[@PotterybyKent]

I'd thought about some sort of magnetic coupling across the lid.

[@junafae]

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

[@PotterybyKent]

A larger volume of plaster is probably the biggest way to stop those

[@jeremygesell]

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

[@PotterybyKent]

That would be quite a setup!

[@nkans]

I think the 3 minute wait is causing problem. I use hands or paddle slow mix while pouring the plaster. With this way, I get bubbles in the top but not inside the mold. Confirmed with chatgpt as well. Proper Mixing Technique: The way you mix the plaster can significantly affect the amount of air bubbles. Mix slowly and steadily, avoiding whipping or fast stirring, which can introduce more air into the mix.

[@PotterybyKent]

The instructions provided by the manufacturer are more reliable than ChatGPT (which is just a next word predictor - my PhD advisor is an AI expert). The instructions calls for both slaking and also indicates that mixing more strongly creates stronger plaster. You do want to avoid whipping air into the plaster - that part is very much true but there is air in there already.

[@nkans]

@@PotterybyKent I can reproduce your whole project idea (line to 3d model with python code) with the same. I can generate necessary python code and take it to printing. Definitely needs code review, but it can take there quickly. It is more than a next word predictor.

[@ronaldlogan3525]

I am wondering if it is really necessary to degas when slaking. Should slake, mix, then degas.

[@PotterybyKent]

Why? I'd be curious to know if there is a reason behind that process.

[@ronaldlogan3525]

It may be that degas in itself speeds up the the plaster to set. By slaking and mixing first, then degas it cuts down on the time used and prevents it from setting too soon. Also degas before mixing is counter productive, since mixing just puts in more air whether you see it or not. @@PotterybyKent

[@PotterybyKent]

Thanks for the thoughts. Mixing likely does add more air, however it seems very likely it could still be net negative by degassing first. I guess this is an empirical question. However, I do think degassing starts the mixing process itself with the agitation. Combined with the vacuum boiling off the water, this procedure overall seems problematic. But fun to try!

[@mattiasfagerlund]

Couldn't you place a motor and a battery on the inside of the container, no need for wires to the outside? There are magnetically controlled on/off switches that might work through the plexi glass. Alternatively, you could put the mixing on a delay. Thumbs up for the overkill.

[@PotterybyKent]

I worry about subjecting the battery to vacuum in particular.Exploding batteries inside a vacuum chamber might be exciting! But also not what I'm going for. If anyone knows of a vacuum safe battery, I might try. The electric motor itself might need some special provisions (eg any grease in the bearings likely will move into places it doesn't want to go, or if the windings are in epoxy, etc). However that one seems more solvable. I guess I could inductively couple energy through the plastic - however I think that is a lot of power to try and get through the magnetic field!

[@hanelyp1]

The biggest problem for a motor in a vacuum will be dissipating heat. Passing wires into the chamber from outside shouldn't be a problem.

[@PotterybyKent]

Good point. My cordless drill does warm up while mixing the plaster now. I have seen this arrangement before (for much larger chambers), but heat dissipation might be part of the design.

[@theabristlebroom4378]

Could you slake the plaster, pour it into the mold, and then put the mold into the vacuum chamber? It might pull the air away from the inner surface at least.

[@PotterybyKent]

It's a good thought, but the 3D printed mold has air cavities on the inside which would likely expand/explode under vacuum. It might be possible if it was printed solid, or if vents were added. For example, resin prints need vents so the uncured resin (and washing alcohol) can come out, so those might work better.

[@akarca]

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?

[@PotterybyKent]

Thanks! Do you mean an STL for the 3d printed plaster form? It is created with my custom software, and if you're interested in trying it out, there is a link in the description for my waitlist (it is still being developed). For plaster, I'm using 70g of water per 100g of dry plaster. And you need about 1kg of plaster for each liter of volume to fill.

[@akarca]

The plastic on the tip of the drill looked like it came out of a 3d printer, I actually asked for it :) I left an email for the waiting list. I'm actively designing with f360 right now.@@PotterybyKent

[@PotterybyKent]

Ah. No it's not 3D printed. The mixing attachment is this one: amzn.to/3Po6QPV. And the M3 driver is this one: amzn.to/44TsHDY

[@TheOrchidDen]

What vacuum are you using? Have a link?

[@PotterybyKent]

It is this one: amzn.to/3qQRWIB

[@coryseaward975]

Just looking at your plaster, I noticed clumping in the pale.. That in its self isn’t good. I typically use Hydrocal. It’s super thin, nano particle size. It’s excellent for plastering molds. Retains it shape and size, no shrinkage and you can get it in different Colors as well. It also comes in different hardnesses for all types of jobs. Just tapping the sides of the mold releases all bubbles and it turns out unbelievable each and every time

[@PotterybyKent]

Can you explain how that might impact captured air and degassing?

[@PotterybyKent]

@coryseaward975 - I see you've edited your comment adding "tapping the sides..". I've seen the same comments about using USG No Pottery plaster so I don't think the type of plaster is the only variable at play. There are many many more including mold shape, mixing strategy, amount of plaster mixed, etc, etc. This is exactly the type of recommendation I talk about in the video which is tricky since the variables are not properly controlled for to determine causality.