Two Part Plaster Mold for Slip Casting

Here I show making a two part plaster mold so we can make a copy of more complex shapes. In particular we can move beyond forms that have a draft angle from top to bottom.
This is the fourth in a series of videos where I'm going to do a deep dive into plaster mold making for slip casting. We'll iterate through different approaches with the end goal of creating some software that can do most of the hard work of mold design for us!
TOOLS & MATERIALS (affiliate)
- 1 Gallon Buckets (plaster form) amzn.to/3OTcQ3o
- Drill Attachment Mixer amzn.to/3OP9iiv
- Murphy Oil Soap (mold release) amzn.to/3MQEWtm
- Bar clamps (for cottle boards) amzn.to/3X2HZTV
- Hot wire foam cutter (for slip well) amzn.to/43QoFeL
- The Essential Guide to Mold Making & Slip Casting amzn.to/3OV2TCw

[@Bob-Is-A-PotterNow]

Excellent! One tool I use that helps with handling internal bubbles and avoiding sneaky lumps is a small kitchen strainer. I pour through that, just as with the slip, and it helps with consistent surfaces. 😁

[@PotterybyKent]

Thanks! I hadn't seen that one before. In these recent experiments I've found that mixing volume has had once of the largest impacts in reducing bubbles (having enough liquid so they don't get sucked in).

[@epidian6240]

Pędzelkiem się dpowietrza

[@theabristlebroom4378]

Kent that moment when you were able to separate the two halfs was so suspenseful! I cheered when you were able to successfully use your two part mold!

[@PotterybyKent]

I wasn't sure it would for a bit!

[@malcolm7436]

This is a great video, and so interesting. Thank you for sharing your method :)

[@PotterybyKent]

Thanks for watching!

[@mikeh2351]

I love mold-making. Good thought process. Another option was to eliminate the bottom foot, smooth-in with clay, and do two piece mold. Then use a round utensil to hammer in a foot indent after pulling out the form. Wouldn't be as precise but it would look better imo than an obviously mass-produced foot. Of course you were doing this as an example so no artistic judgement 😂

[@PotterybyKent]

Yep, just a demo for sure! But it is a great point that you can modify forms to suit your needs like that.

[@esme765]

your videos are awesome, thanks

[@PotterybyKent]

Glad you like them!

[@amania9254]

Another informative video, thanks 😊

[@PotterybyKent]

Glad you enjoyed it!

[@epidian6240]

Forma kiepska , zrobiona amatorsko musisz się jeszcze wiele nauczyć

[@erindemastes]

Cool thanks !!

[@PotterybyKent]

No problem!

[@hanelyp1]

A side by side 2 piece mold could work with the concavity at the bottom. The sides would need to pull away down and to the side. 8:38 happens to have the camera along a suitable pull angle.

[@PotterybyKent]

Thanks. Yes, that is a good point. I think my brain automatically goes to pulling perpendicular to the plaster mating surface. But that doesn't need to be the case!

[@dargon2001]

Muito obrigado pela aula sua voz é calma facilita entender e é muito bela também

[@PotterybyKent]

Thanks for watching!

[@alans1816]

Thanks for a good series of videos introducing mold making. I've been on a similar path, and have some comments. While it's best to avoid bubbles, you can fix them. Make sure the cured plaster is wet, then fill. Wiping before cure or sanding after gives a good surface. When I started, I had problems with mold release. Let it dry before casting. After removing the model, while the plaster is still warm, thorough rinsing of the plaster removes the mold release agent. This speeds up subsequent slip casting, I think. (Not sure because I changed too many things at once, but it's easy.) Sealing with clay rather than glue won't resist buoyancy, but it's a lot easier to disconnect.

[@PotterybyKent]

Thanks for the comments! I tried fixing bubbles in the plaster at one point but had a really hard time getting access down inside the mold (my hands are not small!). As far as clay for sealing molds, I have also tried that but also managed to get a plaster disaster (not sure it was directly related to the clay though)! I have some ideas here that hopefully I'll get to try (and show) in the not too distant future.

[@lumishamei6478]

Thank you for your works..are amazing.. Your videos are very simple to be understand for everyone...I am a begginer to this kind of work and I have a challenge about one mold: can you tell me please what kind of mold must be made for a V 60 coffee dripper?...If can you please make a video about this mold... Thanks and more wonderful videos and moulds 🎉❤

[@PotterybyKent]

Thanks so much! I don't use a V60 myself but have seen a few. It would be a multipart mold for sure!

[@dilian245]

I was wondering, is it possible to make a slip casting with a more complex shame like an indentation of a logo or some other shapes on the model? I have some experience casting things out of plaster, candle wax, and soap, but 0 with slip casting and ceramics.

[@PotterybyKent]

Yes you can! And many people do that, for example having their logo embedded in the foot. One thing you really need to consider are the draft angles and making sure there are no overhangs.

[@mimamima-fr5yo]

انني ممتنة لك على مجهودك الجميل سيدي هل يمكنك ان تعطيني مقدار خلطة الجص من ماء وجص لكي اتأكد ان القالب لا يتكسر اتناء الاستعمال

[@PotterybyKent]

It's 70 parts plaster to 30 water

[@HealthyDiet-pk4kx]

Can we use white cement instead of Plaster? and how long do we keep liquid in the mold before pour out?

[@PotterybyKent]

I don't think cement will work. I keep mine in for about 30 minutes but you need to experiment with your molds and slip to find a timing that works for you.

[@feminissue]

great video! i'm new to this and I'm a little confused, what liquid material is used for the slip? is it just plaster, or is it specifically made for slip casting?

[@PotterybyKent]

The material to make the plaster mold that I use is specifically formulated for use with ceramics. In particular it is USG #1 Pottery Plaster. As far as the slip itself, the clay composition is different than throwing clay, but I haven't dug into the details and I get it from my pottery supply store.

[@caglakaplan1748]

A newbie here as well trying to figure out the liquids.. Did I understand correct that the mold itself is made of modeling plaster (modelgips in Dutch)? Also wonder if this technique can be used to make mold from a clay sculpture and then make a plaster copy to have it lighter and to sand the imperfections afterwards? Many thanks!

[@PotterybyKent]

@@caglakaplan1748 Yes, the mold is plaster - it is specifically design for pottery use like slip casting (other plaster might work but I don't know). One traditional way to do this is to make a plaster mold of a clay form. I think you're asking about making a plaster object from the mold - that might be tricky as plaster likes to stick to itself and is very rigid - but again I haven't tried myself.

[@RoosterPootPottery]

What type of foam are you using

[@PotterybyKent]

Here I used extruded polystyrene

[@Detteermiig]

How much water do you mix with the clay to know that 30 minutes is sufficient drying?

[@PotterybyKent]

Those two are not linked in that way, so this is really two different questions. The first question is how much water is in the slip - follow the manufacturers recommendation (mine is 30% water by weight). Then the second question is how long to wait. That depends on a bunch of factors like your mold, how humid and warm it is, etc. So the best thing to do is test it and see. If the walls are too thin, wait longer next time and vice versa. Similarly for demanding, you can tell when it's ready when its release from the mold. And if it hasen't yet, wait longer.

[@Detteermiig]

Thanks for the quick and detailed response, makes sense :)

[@anjeolaemmanuel3107]

Can you use a plastic bucket instead of wood ?

[@PotterybyKent]

Yes!

[@Alexander_Estrada]

How would you make a mold for the classic vase shape that starts small at the bottom, gets bigger in the middle, smaller at the collar, and then bigger at the mouth? Like this (if the formatting works): \ / / \ \ /

[@PotterybyKent]

Depending on the foot, you would just split it vertically instead of horizontally (eg as I discuss at 4:00). The same overall process would work. If the foot would get trapped, then you might need a 3 part mold - one for the foot and then the 2 sides.

[@mimamima-fr5yo]

هل يمكنك ان تذكر المواد المستعملة مع ذكر مقادير الخلط

[@PotterybyKent]

USG #1 Pottery plaster

[@liejmarose]

can you do plates?

[@PotterybyKent]

Yes. I haven't tried doing them myself yet, but have seen two part molded plates (cast solid).

[@jeannetteelliott6951]

What if my object won’t release?! 😢

[@PotterybyKent]

It depends on why it won't release. For example, did you accidentally create an undercut? In that case there's not much you can do besides break the plaster and try again. Sometimes I've found once things are cool, it is easier to get out. I've also had to destroy the original (usually a 3D printed part). At this point with my 3D printed molds I use a mixture of frozen alcohol and water to slightly shrink the plastic - you might try that as well.