Part 2: How to make a 2-Piece Plaster Mold for Slip-Casting 

You are so welcome!

It's the bomb! :)

I'm so glad I could help you! Good luck!

Glad it was helpful!

Hi Sabrina! Ha! So funny! Thanks for letting me know! I think a lot of professors assign it to their students! :)

Glad it was helpful!

I'm glad you found it so informative! I am a professor, after all. :) So, I really want to deliver quality educational material. I'm delighted to hear you are inspired!

Thank you! Glad it was helpful!

Thanks you! Glad it was helpful!

Thank you! I'm glad to hear you found it so helpful!

Wow! That is so good to hear!

Wow, thank you!

You're welcome! Glad it was helpful! Thanks for the compliment!

Wow! That's so cool! I understand what you mean. It can be difficult to put into words what we do with our hands. And the adding translation on top of it all must be challenging. I'm glad you found this helpful!

Commenting to second this request. I love how detailed and precise you are with your instructions and would love to see a tutorial on how you slip cast.

OK! It's on my to do list!

Ditto on what they said ^^

Glad it was helpful!

Thank you! Glad it was helpful!

Glad you found it helpful!

Thank you!

Thank you! I love the nickname! I'll have to share it with my students! :)

Glad it was helpful!

You're welcome!

You're welcome!

Wow; that's very interesting! I've only used Plaster of Paris and No 1 Pottery Plaster. Thanks for sharing!

You're welcome!

I'm glad you liked it! You only need to wear a mask when working with the plaster in it's dry/powder form. Once it slakes up the water, there are no particulates to breathe. I wear a mask when sifting the plaster into the water, and once the air has cleared (which doesn't take long), I remove my mask. By the time it comes to mixing the plaster into the water, it has thoroughly slaked and, again, there are no particulates in the air to worry about.

Nope. I generally do not slip cast any work to make it solid as it is more likely to blow up in the kiln. The exception is when casting thin pieces, such as wings or flower petals. Those always cast solid. But they are not truly solid due to the was plaster pulls water out of the clay. There is always inevitably always an air pocket in it. But they never blow up because the air pocket is not large enough. Hope that answers your question!

Ahh, that's so nice! What an unexpected compliment on my hair! :) I'm also glad you found the video informative!

Hi jj lad. Sorry I am just now getting back to you. No. Do not use wax. It will permeate the mold and you will not be able to get it off. It will prevent the mold from sucking the water out of the slip, thus turning it into clay.

If you can't get Murphy Oil Soap, use any dish soap you have (or even liquid hand soap for that matter). Since the piece you are casting is metal, you do not need to use a mold release on the lead keel. It will release without the mold-release agent. However, if you are making a two-piece mold, you will need to use a mold release agent on the set-up plaster before pouring new plaster on the mold.

You will have to do some research. I'm not familiar with Alabastrine gypsum. The only other plaster I have experience with is Plaster of Paris, which I have used for slip casting. It works, but not well. You could give it a try and see what happens. Let me know how it goes!

Hmmm. If I understand you correctly, during the first mixing of the plaster, the plaster was thinner while stirring. And the second time mixing, the plaster was thicker while stirring. And both slaking and mixing time were the same. I would attribute this to having colder water in the first mix and warmer water in the second mix. If the water is warm, it will stiffen much faster. (I am also assuming your water to plaster ratios were identical both times) Perhaps the plaster soaked slower during the second slake because your plaster was a little more condensed into an island shape rather than being more evenly dispersed throughout the bucket. Or it might be due to sprinkling the plaster in a little more slowly during your first mix, thus not creating a large dump of plaster that took longer to absorb. However, my guess is the water temperature. This is where you will see the greatest amount of variation. The warmer the water, the more quickly it will thicken, and it really makes a significant difference. Usually the way you sift your plaster into the water does not make that great of a difference. So, even if you sifted in the plaster differently, I doubt it would make that big of a difference. I hope your mold came out well! Making mold of wooden objects can be a little tricky as the wood swells as it absorbs water from the plaster. So, you have to do both sides as quickly as possible. And wood is porous, so it needs to be coated with Murphy Oil Soap before pouring the plaster over it.

@@rebekahbogard3322 Thanks for your quick and elaborate response! It's probably the temperature of the water I used. The first time I took, approx, 2 l, cold water and had it stand for at least an hour to come at room temperature. The second time I took cold to tepid water and used it soon afterwards. Indeed it is likely that the second the time the water was warmer. Next time I'll make a note of the water temperature before adding the plaster to it. Regards, Tom

I would just buy some pre-made cone 6 porcelain casting slip. You can get it at most ceramic warehouses and they often come in one-gallon batches. They also come in a variety of colors. Several white colors along with other colors. Good luck!

You certainly can use modeling clay. But I would recommend water-based modeling clay if you plan to slip cast with it. If you use oil--based clay, the oil in the clay will leach into your plaster and then it would greatly hinder the plaster's ability to act like a sponge and suck up the water (think Scotchgard). When using water-based modeling clay, just make sure you don't let the clay dry out before you add the plaster. If you are not planning to use your mold for slip casting, it doesn't matter if you bed it out with oil based or water based clay. May the force be with you!

@@rebekahbogard3322 Thank you so so much!

Yep. I have used dish-washing soap when I couldn't find my Murphy Oil Soap and it worked just fine. The oil soap is better because it doesn't sud up as you brush it on. But any dish soap should be fine.

Yep, this is how Christmas tree molds are made. You can buy these molds from Duncan if you do not want to go through the trouble of making a tree out of clay and then casting a plaster mold of them. We have a Duncan Christmas tree mold in our studio at school that was donated to us. My students slip cast several every Christmas to sell as a fundraiser. It is interesting how these trees have come back into fashion.

@@rebekahbogard3322 Lovely to hear from you! Your pieces are so beautiful! 😊 I have acquired a motorized, lighted ceramic Christmas tree. Santa plays peek-a-boo as the top of the tree goes up and down. It is the only one I have ever seen. It's adorable! I've been toying with the idea of recreating it. The motor seems simple enough. I guess I can't safely 'copy' this tree by slip casting it... I would have to design a new tree. It sounds like a huge undertaking to me, but I'm still thinking about it. Happy New Year! 🎉

Thank you so much for the wonderful compliment! I’m glad my videos are helpful. Your question is a very good one. If I were you, I would start with press molding since it is likely the plaster is not very good for slip casting and, on top of it, you would have to make your own slip. That is quite a lot of work for such small additions. If you decide you want to experiment with slip casting, you could always drill a sprue hole into your mold later (to pour the slip in and out of the the mold). I have done this before with sphere molds which I use both with slip casting and hand building. I usually press or slump mold if I am making something large, and not a lot of the same thing. Or I will press/slump mold if I want the piece to be more substantial and stronger (with thicker walls). I will also press mold when I hand build a piece. I have a collection of various sizes of sphere molds that I slump clay in and then modify it to make heads or oblong shapes. This speeds up my hand building process and makes it more enjoyable as I can power through some of the basic parts of the animal. And slip casting for this method would take way too long. There is a different kind of speed with slump/press molding since you can almost immediately remove the clay from the piece and then keep working with it. My go-to is slip casting when making a lot of things that are small because it is so much faster once the mold is made. I slip cast a lot of birds and flowers. Also, slip casting is great for super smooth surfaces because the clean-up of a piece can take a long time. Slip casting, when you have the proper supplies, is super fast and easy. But slip casting is not always the best solution. Hope that helps!

Just in case you wanted to experiment with slip casting, you can always buy a pre-mixed gallon. www.dickblick.com/products/amaco-no-15-s-casting-slip/?clickTracking=true&wmcp=pla&wmcid=items&wmckw=30511-1009&gclid=CjwKCAjwy42FBhB2EiwAJY0yQnW2WU-LYrRYKFdiHtlk1kzeYBe3afCgOIBG71-FQLkDkz9X04zxoRoC-YAQAvD_BwE

@@rebekahbogard3322 Thank you so much for such a detailed and thorough answer. :) xxx

The hummingbird was past the leather-hard stage. So it was quite stable and very firm. And the clay bed was very wet clay. Since there is a discrepancy in the moisture content, the two clays do not wat to stick together (wet clay does not want to stick to dry(ish) clay.

The shrinkage depends on the type of slip and how high you fire it. Porcelain slip shrinks about 15%. The slip I used in this video only shrinks about 3 or 4% when I fire it at cone 04.

I'm not sure; it's some sort of plastic. Years of experience makes me good at rolling out clay on non-porous surfaces. :)

The water does not need to be boiled. Sometimes I dilute the Murphy Oil Soap, sometimes I don't. It only takes about 5 min to dry, depending on the humidity level of your environment. I find it is dry by the time I am done mixing my plaster. But I do live in the desert.

Yes, you may add gypsum (plaster) to a plaster mold. But you must coat the interior of the mold with a mold release agent (such as Murphy Oil Soap or Vaseline), then put it together, and pour the plaster in. If you do this, you must make sure there are no undercuts, otherwise you will never get the gypsum to release from the plaster. And you do not pour in the gypsum and then pour it back out after a certain amount of time (like you do when slip casting). When casting with plaster, you will be creating a solid object. Good luck!

@@rebekahbogard3322 lots of love Thank you 😊

Nope. Is there something specific you were hoping to learn?

Same question.

Yes, it should work just fine. Like dish soap, it gets foamy. But it will still work.

@@rebekahbogard3322 hey so um, do i have to wash the plaster mold afterwards? To get the soap off...

@@volstrekt Yes. She mentioned she used vinegar on her sponge and water to clean the soap off.

@@bunch8 i used petroleum jelly.. i made a drip mold btw.. do i have to wash it off as well?

You said it! :) (And so did I!)

Nope. But I do love Murphy Oil Soap! :)

Well, I wouldn't go that far! But you just made my day!!!

Actually, thanks for that comment! I was confused by a previous comment by Bionic Ninja. Now I understand. I didn't realize how much I said that! Funny!