Making Plaster Hump Molds for Drape Molding Clay Slabs

This video demonstrates the second process in making some plaster slump molds- mixing the plaster and pouring it in the leatherhard bowls. If you would like to see the video in which I threw the three bowls of graduated sizes please see this link. • Throwing Three Graduat...
This video begins when the thrown forms are a stiff leatherhard, and are ready to have plaster poured into them. The pieces of clay will be waste clay, as when I have used them with plaster, I don’t want to risk the chance of recycling clay with plaster scraps. Plaster bits would cause pop outs during firing, and would ruin a piece. (Much better, in my opinion to discard!)
I ready my area by covering my table surface to protect it, I have a bucket double lined with plastic bags taped to it, water already measured to reach about ⅔ of the bucket, paper towels, gloves *which I taped additional plastic to lengthen them on my arms… and a dust mask or respirator and a trash can close at hand.
I use COLD water in my bucket, and begin by slowly sprinkling the plaster over the surface of the water. Continue until you have sprinkled enough plaster that it makes a little “mountain” which doesn’t go under the surface. When you have reached the mountain stage, begin a gentle mix without incorporating air. Gently mix with your fingers, and break up any bits which form. Keep an active mixing going the whole time. I find it takes about 15 minutes of mixing for the plaster to be ready to pour. You want to be patient, to make sure it will be starting the process of curing so it will stay in suspension. Pouring it too early can result in a weak plaster and water migrating to the top. Don’t use warm or hot water, as although it will speed the curing, it will also encourage chunks to develop.
For the plastic drape mold, I covered it in vaseline to form a seperator.
In the video, I waited about two hours for the pieces to cure before removing and cleaning them. When taking off the clay forms, I flipped the plaster upside down and set it perched on a banding wheel, and then loosened up the clay around all the edges. Once removed, I scraped with mudtools scrapers and kitchen scrubbies. (see my googledoc links or my Amazon storefront for the ribs and shredders.)
I used a Mudtools Shredder to remove uneven corners. (I blanketly call them “surforms” which is technically a Stanley product I first used over 30 years ago. Mudtools are a great option made for clay- but essentially have the same mechanics of the surforms)
I show a few other types of drape molds, and you can find links below to some on my googledoc, or in my Amazon Influencer Store.
Check out my Amazon Influencer Store where I have recommendations for some of my favorite tools and products many of which I use in my videos- which are available on Amazon. www.amazon.com/shop/karanspot...
For the great tools I used in the video (including the mudtools long green platter rib and the yellow rib) check out this link to a live Googledoc I continually update with lots of my favorite tools on Amazon using my associate links! (also some non-amazon suggestions on here!)
docs.google.com/document/d/1X...
In this video, I am using standard 153, a nice midrange firing stoneware in my classroom.
I am a public high school ceramics teacher at William Mason High School in Mason, Ohio. (Mason is currently the largest high school in Ohio!) As a potter, I have been working in clay for over 30 years, and I have been teaching for over 28 years, the past 20 have been specifically teaching high school ceramics. I love what I do! I have my own studio in my home basement, where I work on my personal pottery for my Etsy site; www.etsy.com/shop/KaransPotsA...
I started my KZbin channel to make videos to help my students if they are absent and miss a demo, or if they would like to explore more advanced techniques independently. I have been amazed to reach such a wider audience than my own students!
Please subscribe to my channel if you would like to receive channel updates, or would like to learn more about clay and studio processes!

[@pizzatopia]

plastic bag liner on mixing bucket is a great pro tip! nice vid, thank you!

[@apexpotter]

Nice job! Thanks for sharing this with us.

[@gtrapper3745]

Very nice demo, thank you!!!

[@valgoodridge1810]

Thank you for this video. I am returning to hand building after many years on the wheel. I have wanted to create some plaster hump molds but have been hesitant to try. You have inspired me to give it a go.

[@KaransPotsAndGlass]

Yay! I’m so glad! :)

[@julipolito7761]

Thanks Karan! 👍. Juli, Tucson, AZ 🌵

[@jackiemorrison6024]

You're one of my fav teachers... awesome video

[@KaransPotsAndGlass]

Awww- thanks, Jackie!

[@lwd3132]

I so appreciate this! THANK YOU. :)

[@suzannemccaslin8420]

Great instructions! Ready to start making some hump molds. Thank you!

[@KaransPotsAndGlass]

Have fun!

[@carinwiseman4309]

Ask a farm vet for long gloves that they use for horses and cows...to palpate pregnancy..haha. these gloves are full arm length. Your videos are the best how-to videos for beginners I have seen. Thanks for making these available to the rest of us who are not your students. Hope they know how lucky they are to have you for a teacher..WOW!

[@lredpathworlund5071]

Nice video...tried it today using a glass bowl as a mold form. Didn't have any vaseline so thought coconut oil might be a good 2nd choice. Wrong! It did not release. Will give it another go...Thank you!

[@billmaslak8963]

Thank you very much. Very informative. Well done to.

[@KaransPotsAndGlass]

Glad it was helpful! Just know- I found nice long rubber gloves on Amazon since I shot this!! 😂😂

[@sixtaleocadio9892]

Hermoso tu trabajo. es la primera que veo de ti me encanta y voy prender gracias gracias por compartir

[@TheMarshain]

Yessssss! I wanna do that, too!

[@KaransPotsAndGlass]

It's SO worth the effort!!

[@violetsands]

If you use a flexible trug to mix your plaster, then when you are finished you let the plaster dry in the trug and then flex the sides which will release the plaster bits and then put into the refuse - that way you use less plastic which is better for the environment :)

[@KaransPotsAndGlass]

Very true- but my main concern is keeping all plaster bits out of my sink (as would inevitably happen during washing) when I have to wash- that's the main reason I use the bag! :) But- great suggestion!

[@marciacunningham5877]

Johnsons paste wax makes an excellent mold release. Thin with no texturing. Michael

[@runningbear4666]

this will rock

[@KaransPotsAndGlass]

I demonstrate in a future video how to drape slabs on the molds and use footrings... so they don't rock! :)

[@runningbear4666]

KaransPotsAndGlass this will be a fun day watching you thanks for the heads up

[@carlasmith2105]

Very nice. You make it look so easy! ;+)

[@sabinekeil7584]

Great tutorial. Very useful. I've only made 1 mold for drying sludge clay. I left it in the plastic mold (without using any release agent...cause I didn't know better) for a couple of weeks. As it shrunk it released from the sides and just popped out. It was perfect. Would this be the norm...if I just left it in there? I need to make larger drying bat and wanted to use a large stainless steel container. Can I? If so, should I use vaseline or do you reccomend something else? I also want to make some slump molds. Can I make thick walled but hollow molds so I can use inside and outside? So many questions... Thank you!

[@robinbergson959]

how long do u let the plaster sit in the mold before getting it out...how long do u let the plastic set up and dry before you can use it?

[@slice51]

Hi. Thanks for this video. Did you need to line the clay bowls with something?

[@ramseyridge]

Hi! Thanks for the video! Could you add colored powder pigments into the plaster mix as a way to color it without painting it?? Or would that disturb the mixing process?

[@KaransPotsAndGlass]

I believe you can if the ratio isn’t too great, although I have not!

[@frustrating123]

Hi Karen, When it comes to putting my Dom 20 clay slab on my new slump pottery mold made from plaster, will I need to use a release like Vaseline or?

[@KaransPotsAndGlass]

No- plaster by it's nature is porous. No release needed. If your plaster is dry, it should not stick!

[@vickiserva9322]

How long before you can use molds after making. Information very helpful,thank you.

[@KaransPotsAndGlass]

Depends on how quickly they dry! I usually give at least 48 hours before I begin to use them!

[@zoeyatsivisacowitz8272]

can you please specify what type of mold release you use rather than vaseline / thank you

[@KaransPotsAndGlass]

When pouring plaster over another object- I would normally use petroleum jelly, (vaseline) but I have heard of others using Pam or kitchen spray. Even W-D 40. I usually avoid those items, as I don't like getting it into my clay. But, it may work fine!

[@lisatello5464]

Can I make a plaster mound of a baked clay doll I made? I’m afraid of under cuts in the face features?

[@KaransPotsAndGlass]

Sure, it’s possible, but you’d need to do a multi- part mold due to undercuts. I don’t have any videos showing that, but you should find some on KZbin I’d think! :). Good luck!

[@amykrenzer4646]

Thanks! Can you tell me, if I use a flexible plastic bowl, would I need a release agent?

[@KaransPotsAndGlass]

Yes, I would still use a release agent, just to be safe with a new material. It's possible you may not need it- like if it is silicone and super flexible, but without knowing entirely, I would use one!

[@micheledickey4066]

I don’t have a huge bag, only a few pounds. Is there any way to measure the water and plaster ratio so I know I’ll have enough?

[@KaransPotsAndGlass]

I’m sure there’s a formula online somewhere if you do a search! I just don’t have one handy- or the time to check tonight! :)

[@chrissykelly]

Thanks so much for these videos! I was wondering if a plaster mold is made in a plastic bowl using Vaseline as a release, does it affect the absorption quality of the resulting plaster mold? Thank you again, these videos have been great to watch!

[@KaransPotsAndGlass]

No it doesn’t- you can always wash it a bit once it hardens if you find any residual Vaseline!

[@chrissykelly]

@@KaransPotsAndGlass Thank you so much for responding, you are so helpful! I've been binge-watching all your videos and am loving them. Your students are very lucky! Thanks again!

[@godoggo407]

Love your videos. What proportion of water and plaster?

[@KaransPotsAndGlass]

Hi I explain it in the video, and this is what I have written in the video description below the video: I use COLD water in my bucket, and begin by slowly sprinkling the plaster over the surface of the water. Continue until you have sprinkled enough plaster that it makes a little “mountain” which doesn’t go under the surface. When you have reached the mountain stage, begin a gentle mix without incorporating air. Gently mix with your fingers, and break up any bits which form. Keep an active mixing going the whole time. I find it takes about 15 minutes of mixing for the plaster to be ready to pour. You want to be patient, to make sure it will be starting the process of curing so it will stay in suspension. Pouring it too early can result in a weak plaster and water migrating to the top. Don’t use warm or hot water, as although it will speed the curing, it will also encourage chunks to develop.

[@rowanmacmillan]

wondering how to put a pedestal on..

[@KaransPotsAndGlass]

Hi Rowan- I actually have just made messy ones- they’re never that fancy… but, here’s how I did them! I first would rough up the connection point of where I want to put the new attached plaster. I do it after the initial pour has set up some maybe an hour or so and I mix up new plaster. When I mix up the new batch, I get it to the pourable consistency and then I lightly put a skim coat over the area that I just roughed up. When it starts to become a bit more solid like a pudding, then I scoop it out into a pile in the area where I want the pedestal to be then I should’ve mentioned I have it on a turntable or even a wheel if you’re careful not to get it all over everything. Then I start the rotation on the pedestal or the wheel, and with my hands being a little bit wet and perhaps a rib, I just start forming it into a base pedestal almost like a mushroom stem. Making sure it is wide enough to support it and it won’t be wobbly. I hope that makes sense! Let me know if you have any questions!

[@tracyevans7345]

Could making another mold in a small bucket and fixing it with wet plaster work?

[@KaransPotsAndGlass]

Maybe? I mean I add wet plaster to set plaster- so I suppose if you roughed up both set surfaces shortly after pouring it may work!

[@teddyonwheels]

What does it call for the metal throwing wheel?

[@KaransPotsAndGlass]

Are you asking about the cost of a potter's wheel?

[@dancingangel411]

Can you share how you were able to create the pedestal/mushroom mold that you showed briefly at 13:50? How did you add the bottom part? Thank you!

[@KaransPotsAndGlass]

I made that so long ago, I am trying to remember! Lol- I waited until the initial poured mold was cured, then I used more plaster, and scratched the surface of the poured mold, and then waited to add and sculpt the foot on that when the plaster was setting up and thick. Since I scored the first plaster part, the new plaster adhered well to it. Then, with it on a turntable, I just took my wet rubble gloved hand and kinda rounded and sculpted it before it was totally set.

[@dancingangel411]

@@KaransPotsAndGlass Thank you!

[@tertianaude5402]

Thanks for the video. Is there any particular reason why you would not recycle this clay mold?

[@KaransPotsAndGlass]

Yes! I didn't want to take any chance that a bit of plaster would get mixed up with the waste clay. Plaster bits will be devastating to a piece if they end up fired.

[@tertianaude5402]

@@KaransPotsAndGlass O okay. Got it, thank you.

[@lindaknox7725]

I'm a beginner potter and so enjoy the information you provide. I am wondering how you decided how much water you needed for these molds. Did you fill each one first with water? Did you have very much plaster left in your bucket?

[@KaransPotsAndGlass]

Hi Linda- honestly, I don't figure it out precisely before I make the plaster. I normally have more molds ready if I have extra I pour it in!! If I ever do have too much plaster, I just dump it. :). The only time I figured out exact measurements was when I was making the damp boxes for ALL my students last fall... I had to use a formula to determine how much plaster I had to purchase. I just looked up a formula of weight of plaster to volume. I don't recall what it was... but found it in a google search! ;)

[@lindaknox7725]

Thanks so much for the reply. That helps me lots. Now I have to find the plaster

[@tracyevans7345]

I’ve got a huge bag of gypsum.. can that be used to make a mold?

[@KaransPotsAndGlass]

I don’t have experience with gypsum alone- typically I’ve only had it as an ingredient within plaster mixtures. Maybe try a tiny mixture of it to test?

[@ncmy4kids]

Thanks again for all your videos! I have another question though. If I use a bowl from my kitche to make a mold, is it still safe to eat with after I wash it like normal?

[@KaransPotsAndGlass]

I’d say it depends on the material. I’d say if you can thoroughly clean the surface free of all residue you’d probably be fine. But- be careful if you use a non-flexible mold with plaster- it may not come out! Like glass, metal, and ceramic might be difficult to remove the plaster. Plastic with a little flex is best. And don’t forget the release agent!

[@ncmy4kids]

@@KaransPotsAndGlass Thank you!!

[@elenarakochy2492]

I didn't see you do anything to tap out the air bubbles once the plaster was poured into the molds, but I've always understood that to be a necessary step. Is this something that gets perfected over time so you don't need to worry about air? Thanks!

[@KaransPotsAndGlass]

You’re right!! I didn’t show it- but I think I probably did do some gentle tapping off camera. But, because the molds weren’t truly rugged, I believe I literally lifted and tapped the table to help the bubbles raise and pop! Otherwise, I would lift and tap the mold itself!

[@elenarakochy2492]

@@KaransPotsAndGlass Thank you. This is very helpful!

[@kevinleong4467]

If you use those tools for scraping the plaster off can you still use it for throwing or do you only use it for plaster now?

[@KaransPotsAndGlass]

I use the same tools for both clay and plaster. BUT- that is just ME, and I am responsible and clean them thoroughly between uses. I don't believe I would ever trust my students to do that!! ;)

[@ereniko7994]

Hi, I wonder how I can remove it from a wooden mold with the upper and lower sides open?

[@KaransPotsAndGlass]

Plaster will adhere to wood unless a very good barrier or separator is used. If something like vaseline won't work, you could try layers of plastic wrap perhaps? I can't really picture what your mold looks like though!

[@ereniko7994]

@@KaransPotsAndGlass Thank you so much. For your help. Can it work if I try Arabian soap or Oil? My mold is a shaped prize mold.

[@KaransPotsAndGlass]

@@ereniko7994 No idea about those- sorry!!

[@ereniko7994]

@@KaransPotsAndGlass Thank you so much 🙏🏻

[@patspotpage]

I don't recall you telling why you are throwing away the clay molds. It may help some to know what happens if you get plaster mixed in with your clay and then fire it!

[@KaransPotsAndGlass]

If plaster gets in clay, during the firing process, it will expand and create pop outs, wherever it’s in the clay wall. You can have giant chunks pop out because of the expanded plaster bits.

[@claytonwason3488]

Isn't the Vaseline defeating the purpose of the Mold?

[@KaransPotsAndGlass]

Defeating the purpose? I don't believe I understand your question. The vaseline allows the mold to release from the plaster. Without the release agent, sometimes the plaster will be stuck to the mold and will ruin the mold and the plaster you pour into it. (It really depends on the material that mold is made from- in the case of my plastic oval bowl I used, it was a very stiff plastic with no flexibility- it would have ruined it without the vaseline to release the plaster.). Did that answer the question?

[@claytonwason3488]

@@KaransPotsAndGlass Vaseline Is a water repellent? Isn't a Mold supposed to suck moisture out?

[@KaransPotsAndGlass]

It depends on what your mold is made of. Many different objects can be used as molds... like my plastic bowl proved to be a handy mold to make the hump mold from it. PLASTER molds ARE designed to suck out the moisture from the clay slip you pour in. However, I am not using slip. I am pouring plaster directly into a mold that doesn't have porosity like the plastic bowl. That being said, I don't have to use a separator on the leatherhard bowl that I am pouring plaster into as that will release without issue.

[@tom76143]

,