Can I slip cast holes for the Rice Grain effect directly into my pot? 

Thanks for the ideas. A drill guide could be useful. I think I'll also need something to set the depth of the pin I insert eventually for consistent results. Casting the pin holes directly in the plaster would save the post-processing, however I'm not sure exactly how I might do that and still demold the plaster afterwards or how I could attach them to the inner and outer molds of Shape Cast. But that could be worth thinking more about.

​@@PotterybyKent So a simple-ish way to cast holes in place would be basically the same thing you've done with the plaster mold, ie a shell with a bunch of holes in it that the pins go through with a light interference fit, except now the shell is the mold for the plaster instead of the plaster itself. I acknowledge the complications, that the plaster seems like it leaks more easily than the slip, and that the pins would have to go through both the inner and outer molds without leaking too much. If the pins are independent, then this means the outer mold can double as a jig for holding the pins at the correct depth for the slip casting, if designed that way. If you do end up wanting to gang the pins together, then the holes would have to be made parallel to each other in groups eg perpendicular to the faces of a polyhedron, and each face of the polyhedron corresponds to a group of pins that are attached to each other, and the jigs attaching those groups of pins together would be separate parts unrelated to the mold.

Ah I see. Yes adding all of the holes from the out to inner mold would seem very likely to leak plaster. The gasket I use between the mold parts right now is very much needed otherwise even the "flat" 3d printed surfaces leak a lot of plaster. Ganging the pins together is also an interesting thought although that would end up with holes in the final piece that are not normal to the surface. But that might be so subtle to not really make a difference in practice.

Can you say more about the clay body you use? As I allude to, I think the porcelain like nature of my clay is making it more susceptible to cracking.

​@@PotterybyKent I use 3 types of clay typically. Cone 6 "Salmon White" from georgies pottery supplies, and Natural White ^6 Dry Stoneware Slip (SSD600) from Georgies. Both of these are very pliable, elastic, and have high water absorption. I add either sodium silicate or darvan 7 to my slip, just the recommended amount. Both of these are smooth and grogless. I also throw some backyard clay. Hard to compare to this one. It does not pass the elastic bend test that a lot of folks do, and it takes a lot of babying to prevent cracking. I am able to poke holes of about 1/4th or larger only when its still very pliable, far before leather hard. If i do it any closer to leather hard, i will blow out similar to yours, but it has a very grainy structure to the chunks that fall out.

Thanks, that's good to know. My clay body is not very plastic being porcelain like. However, it's the only dry clay my supplier has tailored for slip casting. I guess maybe I'll need to dive into clay body formulation.

Thanks! Yes my next step will be to scale it up and add lots! of holes, and then to a bigger form.

@@PotterybyKent I'm curious if you wanted to make this work with many more holes if you could do something like adhere plastic dots to the inside of the mold so the plaster will not make contact with the slip in that specific area, in theory making a hole on the inside...

What theory indicates it would leave a hole? I't might be thinner but I don't think there would be a hole since the slip will still settle out (like it is doing on the nails).

@@PotterybyKent Hmm, the seal wouldn't block the plaster from sucking water from the slip? If the hole was large enough

It might stop the plaster from absorbing water. But the water also isn't absorbed into the nail and as I show it ends up coated with a layer of slip. I'd imagine a similar thing will happen. However I've never seen it tried so that is just a guess based on this experience. What it might do instead is create an area that is thin that might be easer to clean up - however I'd really not have that extra step if possible

I doubt wax or paint on the mold would have the intended effect, but if you try let me know. For my intended form, I can't reach the inside. If I could then it would be easy to clean up any of these with a sponge. There isn't adhesion to the nail, just deposition and I'd imagine changing to wax would result in the same effect (or I don't see why it would be different).

Interesting - if you have a link for a video or picture, I'd be curious to see it or something similar. As far as over deflocculation - that is why I switched to Darvan 7 very early on. There's not much worry for that.

​@@PotterybyKent I don't have any pictures or videos of the process, sorry. Mostly because I have slip all over my hands when I'm doing it. I also don't know how useful it is given it's quite a different use-case. Maybe give it a try? After drain, try filling the clay body (still in mold and with the needles in of course) halfway with water and swirl it around. The "settled" clay won't easily dissolve again, but the thickened slip will more easily dissolve. The Darvan 7 sounds pretty nice. I also use a polymer-type defloc, called sodium dispex. Haven't seen Darvan 7 here in the UK.

Understand hands being full doing it! Thanks for the extra info.

Great idea. And perhaps, on top of that, try using wooden pins, or bamboo. I could imagine that turning them around when taking them out would create nice round holes. Just a thought…

Thanks! For my sphere form I can't actually reach inside - the hole on the top is too narrow. However for other shapes I think that would work. You could also clean up the holes with a sponge that way as well.

@marcelzwiers - if the hole was a bit undersized, I could imagine spinning something with texture might indeed do that. I think the problem with wood or bamboo is that it would absorb the moisture from the slip and thus create a build up. So that would only really work if they could be pulled from the inside as suggested above.

One other thought along these lines. If the form allows for pulling the pin from the inside, then the hole in the plaster doesn't need to go all the way through the mold. And if you drilled them all to the same depth, then that could facilitate getting all of the pins to be sticking through the slip the same amount.

1. Preparation of the Silicone Balloon: Create a silicone balloon with pins arranged according to the desired design pattern of the perforations. Melted wax is poured into the silicone balloon. Once solidifies, it forms a rigid internal structure that maintains the balloon's shape and supports the pins.

2. Slip Casting: The wax-filled, pin-outfitted silicone balloon is placed inside a two-part plaster mold designed for the gourd lamp. The positioning of the balloon ensures that the pins protrude towards the mold's inner surface, touching it firmly to create designated points for perforation. Porcelain slip is then poured into the space between the plaster mold and the silicone balloon. As the slip fills the mold, the pins penetrate through it, reaching the plaster mold and ensuring the formation of perforations at the intended locations (mold rotation might be required)

3. Creation of Perforations and Removal of Internal Structure: After the slip reaches a leather-hard state, indicating that it has partially dried and solidified, the piece is gently heated to a controlled temperature that melts the wax inside the silicone balloon without damaging the porcelain. The melted wax is carefully drained out through a specifically designed hole in the gourd lamp, also intended for installing the lamp's electrical components later. A protective tube is inserted around the edges to ensure the edges of this hole remain intact and are not damaged during subsequent steps. This tube acts as a safeguard, preventing the shrunken silicone balloon from coming into contact with and potentially damaging these edges during removal. With the wax removed, a vacuum is applied to the silicone balloon, causing it to shrink. This shrinkage facilitates the gentle removal of the balloon through the same hole, avoiding damage to the delicate, newly formed perforations and ensuring the integrity of the hole edges is maintained.

4. Finishing Touches: To finalize the piece, the external surface of the slip cast is carefully trimmed. This trimming process cleans up imperfections and ensures that the edges around each perforation are sharp and well-defined, revealing the intricate pattern designed in the initial stage. The lamp then undergoes the standard processes of drying, bisque firing, glazing (if desired), and glaze firing, as per traditional ceramic finishing techniques.

Perhaps we could consider patenting this method. 🙂

I don't see how metal would work. Concrete is interesting but I'm not sure how it might be better. I've heard of people using bisqueware for press molds - not sure if those would work for slip. Also, CNC on plaster apparently cause issues (something I heard from a ceramics plaster expert).

I am using metal molds because I need to heat up the slip while its drying and the plaster was cracking for me. Concrete is not really better than plaster, but it's easier to work with for me, and can be CNC-ed. But anyway, great content. @@PotterybyKent

Alternatively, create a composite plaster sphere mold consisting of 5-6 parts, embedding small silicone cylinders into its inner surface :-)

Having the wax burn away is an interesting idea. The problem as I alluded to elsewhere is that I'm pretty sure there would be clay buildup even on wax. Not a lot, but not zero. As far as multipart molds, I want to get there with Shape Cast. The idea of a lot of small inserts is interesting but I think has the same issue as the wax.

It's not drying. It's just depositing there (falling out us suspension I guess) like slip on the side of a plastic bucket or the PLA in my funnel, and plastic is hydrophobic.