Clay Horse's Finish or Silicon Mould

[Sean Fisher]

Hello,

I'm totally new to ceramics, but I have this problem I'm not sure how to solve. I want to finish these horses and glaze them, but I also want to take a silicon mould from them so I can make new clay horses. 

I've read about this online and I think one method is to take a mould the way they are now ( I also need to fix them) and then use that silicon mould to make plaster cast and then once I have that cast create a plaster mould so I can use slip porcelain. 

 

But if I do that I heard that the mould release will be on the surface of the clay and I won't be able to then glaze those ceramics and they will be left unglazed. I really want to continue this project and I don't want to have to sculpt the horses again.

 

So if you can let me know what you think is best then please let me know! 

 

Attached is the photo of the two horses. 

 

https://imgur.com/a/TcIRuJ7

[PeterH]

I suspect you are under-estimating the difficulties of making a slip-casting mould for a shape like a horse.

It will have to be a multi-part mould, and probably with a lot of parts. I've never tried anything like that, but it could easily be:
- 6 parts or so to handle the legs [1]
- at least another 2 for the back, neck, mane  and head
- probably another 1 or 2 more for the ears
- maybe another 1 or 2 for the tail

Not a task to be undertaken lightly. Which is why some horse modellers don't even try.

Maybe I really can DO this! https://westerlydesign.com/2009/07/02/maybe-i-really-can-do-this/

The Caprice horse sculpture has 9 separate molds. ... There are separate little square multi-part plaster molds for the head, each ear, all 4 legs, the main body, and the tail. Some molds have just 2 parts and some have several. Caprice’s body has 4 mold pieces you pull away from the cast greenware part.
 

Not ideal (needs some skilled per-horse assembly), but makes the mould making and use a whole lot easier.

[1] Plaster Mold and Model Making, Chaney and Skee
fig 207 Cross-section of the legs of a horse shows a six-piece mold is necessary for the legs alone.

[PeterH]

>But if I do that I heard that the mould release will be on the surface of the clay and I won't be able to then glaze those ceramics and they will be left unglazed. I really want to continue this >project and I don't want to have to sculpt the horses again.

I don't know what parting agents work with silicone, but these three sealants you can apply to green-ware that will burn out during bisque fiiring.
- wax resist
- shellac
- acrylic medium

Etched in Clay: How to Make Beautiful Relief Surfaces with Shellac Resist
https://ceramicartsnetwork.org/daily/article/Etched-in-Clay-How-to-Make-Beautiful-Relief-Surfaces-with-Shellac-Resist

[Jeff Longtin]

I have made multi part molds, over the years, and they usually take much longer than expected. 

I've never poured silicon against unfired clay. My preference would be to fire the pieces so they have some durability.  You will see some shrinkage but durability would be the benefit.

As the shapes are organic in nature, you probably won't find the perfect parting line, until you've made at least one pouring, per shape. As a result you will be doing lots of pouring.

Good Luck!

[PeterH]

@Jeff Longtin Would you use a 2-piece silicone mould for a horse, and would you cast it as such or use a cut-out technique (e.g. https://www.youtube.com/watch?v=xLgd2pj-XCs)
If the latter, could you get a green-ware master out undamaged.

If you intended to make a multi-part plaster mould from a  casting from the silicone mould what are the merits of
- a green-ware master
- a fired master (bisque or vitrified?)
- a resin master

 

[MAO]

You can’t use slip in a silicone mold. You can use silicone to make a rubber master mold with a rubber master horse. Which you then use to make plaster production molds. The trick is doing the rubber mold exactly like your plaster mold will be. This means accounting for undercuts and all your mold parts. Generally I do at least four parts: each side, a “hat” for the ears and a part or two between the legs. The pour hole is usually in the belly if the horse is standing (also the reason you might do two separate pieces between the legs). You don’t need to cut it up into so many pieces. That’s done in bone china casting because it will not bend as much as earthenware slip. Also tends to have a clay memory and warp. Low fire slip is generally more flexible and with harden as you place it.

A couple notes:  make sure you seal your original before covering it in rubber. Do not use raw bisque! I use urethane rubber (RU440 from Silpak in California). It has a 24 hour cure time, and you can get away with not using a pressure pot when making the mold. A vacuum pump to suck out some of the bubbles is useful.

Remember: make a rubber version of your horse in the rubber version of your plaster mold. You’ll be pouring the plaster around that. You don’t need to use any mold release rubber to plaster. You need release rubber to rubber and plaster to plaster. You will be replacing each rubber part, one at a time, pouring plaster into the space, cleaning up the part and coating with release (not where it touches the horse! You need that unsealed to absorb moisture from the slip). 

[MAO]

In looking at your horses, the one on its side is pretty simple. Just make sure there are no 90 degree angles on the sides. Give that a slight outward flair and you can probably do two parts for that one (top and bottom). The one on its back has one major problem area: the leg over the head. That leg I would cut off and mold separately. Otherwise it looks like a 4-5 part mold.

[Jeff Longtin]

@PeterH

Just viewed the video. That looks difficult. I know it's possible, but I've never done it. 

Master material: I prefer that a master model be in a permanent state.  Greenware is extremely fragile. (It's fragile sitting on the table and it's fragile in the molding process.)

I've made molds, from leather hard clay models, and I've made molds from bone dry (greenware) models. Both increased the challenges of mold making. Especially when making a multi part mold. 

Something in a permanent state; plaster, rubber, resin, allows me time to consider my options. It also allows me to make mistakes. Nothing worse than finding out you set the parting line poorly only to have the model destroyed in the process.