Bisque firing piece inside mould?

[tkw954]

I'd like to produce a ceramic piece with a relatively complex shape and would like to use a process like lost PLA casting, but with slip.  I'd 3D print a positive in PLA, cast a negative mould around it, burn out the PLA in a low temperature kiln, and slip cast using stonewear slip.  Then comes the hard part: this would be a one-piece mould that would be broken apart to remove the cast piece, but I doubt greenwear would survive the violent demoulding.  Can I bisque fire the piece inside the mould in order to support it? I doubt that a standard plaster mould would be appropriate, but is there some other investment material that would be possible to make a mould absorbent enough for the slip-casting process that would either survive a kiln firing, or (better) slowly degrade as the clay matures? Any ideas?

[PeterH]

Technology

This last posting in this thread may be of interest:

It mentions work on lost-wax kiln-casting of glass articles using a mould of 1 part silica and 1 part potters plaster. I've no idea if you could cast (and then fire) stoneware in this.

*  I would worry about getting all of the PLA out before you pour the stoneware slip, as the casting-surface left would have to  be very absorbent.

* Stoneware may be less forgiving in terms of thermal expansion/shrinkage than glass.

PS I assume that the shape of the object means that using the PLA master to make a multi-part plaster mould is out of the question. A picture would be interesting.

[PeterH]

Another approach, if a multi-part mould is feasible:

3D Printing a Mold for a Slipcasting Mold
https://www.instructables.com/3D-Printing-a-Mold-for-a-Mold/

Picture of 3D print, and the plaster mould cast from it. A one-off process as the 3D print is typically destroyed to extract the plaster mould.

[tkw954]

Thanks @PeterH.  I think your first link *may* work, as long as the silica in the plaster doesn't kill its absorption properties.   Luckily this won't be a mass-produced piece so I can take extra time for casting. Easy enough to test.

Unfortunately, the piece I have in mind has a lot of undercuts and interior details that won't be possible to produce using a reasonable multi-part mould.

[PeterH]

Good luck, and let us know how you get on.

My big worry would be residual PLA clogging up the casting surface, and to a lesser extent the silica sintering onto the stoneware during the bisque firing (may have a tweak for that).

If this doesn't work I suspect an intermediate silicone rubber mould might help. Perhaps used to cast a "crumbling" plaster mould that is only fired once. I think I've seen plaster+sawdust mentioned  somewhere. [Perhaps when casting a hollow cylinder with a sacrificial mould-part forming the hole, which cannot be removed before firing without damaging the casting.]

PS Always wanted to cast an umbilic torus!  https://tinyurl.com/sc8yctts

 

[tkw954]

The description in this (relatively clearly worded) patent may be applicable: https://patents.google.com/patent/US5266252A/en

Quote

A wax pattern for a shell mold is provided, and an aqueous mixture of a calcium sulfate-bonded investment material is applied as a coating to the wax pattern. The coated wax pattern is then dried, followed by curing to vaporize the wax pattern and leave a shell mold of the calcium sulfate-bonded investment material. The shell mold is cooled to room temperature, and a ceramic slip is poured therein. After a ceramic shell of desired thickness has set up in the shell mold, excess ceramic slip is poured out. While still wet, the shell mold is peeled from the ceramic shell to expose any delicate or detailed parts, after which the ceramic shell is cured to provide a complete, detailed, precision ceramic article without parting lines.

 

[Callie Beller Diesel]

I am not the most versed person in plaster, so someone correct me if I’m wrong, but isn’t plaster made by slaking lime, which is essentially heating it up to drive water off?

Wouldn’t firing it too hot make the mould crumble? 

I know from experience the silica/plaster mix is pretty friable after firing, and moulds made with it for glass slumping are single use.

[tkw954]

@Callie Beller Diesel I think gradual crumbling at high temperatures is fine (even good from a mould release viewpoint) as long as the clay has started to mature enough to self support at that temperature. I'm more concerned with explosive "pops" due to trapped water. Maybe it's a matter of making a porous enough plaster so that the water can escape gradually.

[PeterH]

49 minutes ago, Callie Beller Diesel said:

I am not the most versed person in plaster, so someone correct me if I’m wrong, but isn’t plaster made by slaking lime, which is essentially heating it up to drive water off?

Wouldn’t firing it too hot make the mould crumble? 

I know from experience the silica/plaster mix is pretty friable after firing, and moulds made with it for glass slumping are single use.

I shared your concerns, but the referenced article on glass kilncasting seems to indicate that the process used addresses the issue. There is some discussion leading up to:

The sacrificial mould is broken off the casting:

[tkw954]

OK, I'll try it with a plaster/silica mould and run a cycle with holds similar to the glass cycle but stopping at cone 06.  I'll try a shell mould as well as a vat mould.

[Callie Beller Diesel]

@PeterH I was more concerned about the step with the PLA burnout. What temperature does that happen at? If that firing is hot enough to denature the plaster, there could be issues with its absorbency and ability to cast with slip.

@tkw954 I have worked with the silica/plaster blend for glass slumping and fusing. The silica doesn’t fuse to the piece, it’s to make the plaster more refractory. Some of the material can be difficult to clean off glass, which melts much more than bisque will, but it all comes off with water, a scrub brush and some elbow grease. I suspect any residual silica on a bisque piece would rinse away. And you probably should do water cleanup for that job, come to think of it. 

The only steps in that firing cycle that pertain to the plaster are the first two. The rest are concerned with heating and cooling the glass enough so it flows without bubbles or voids forming, and annealing it so it doesn’t devitrify or fracture. That AFAP is glass speak for “open the kiln lid.” The piece in the image is pretty thick, which means a lot of thermal mass, thus 6 hours at 900. Those things are going to have no bearing on either plaster or clay, and you can omit those parts.

 

[Piedmont Pottery]

There are some soluble 3D filaments.  Perhaps print with one of these, form your mold, and then dissolve the printed part, no firing necessary

[tkw954]

Yeah, @Piedmont Potterysoluble would be better.  I'd forgotten about that.  That might solve all the issues with burnout and absorption (assuming it washes out cleanly).

[PeterH]

2 hours ago, tkw954 said:

Yeah, @Piedmont Potterysoluble would be better.  I'd forgotten about that.  That might solve all the issues with burnout and absorption (assuming it washes out cleanly).

Could you cast the mould/shell against it?

[Piedmont Pottery]

PVA filament prints take several hours to dissolve in water, but since the plaster curing reaction in exothermic, there could be some chance the print might partially dissolve in the moisture of the plaster before it cures.  PLA is soluble in solvents like ethyl acetate or concentrated sodium hydroxide, so that might be worth trying if PVA doesn't work out.

[tkw954]

4 hours ago, PeterH said:

Could you cast the mould against it?

Good point, I don't have any PVA experience, so don't know if it would dissolve in the time it takes the plaster to harden. 

 

3 hours ago, Piedmont Pottery said:

PVA filament prints take several hours to dissolve in water, but since the plaster curing reaction in exothermic, there could be some chance the print might partially dissolve in the moisture of the plaster before it cures. 

Good to know.  Might be a good idea to brush on thin shell layers to encapsulate the PVA without it getting too hot.

3 hours ago, Piedmont Pottery said:

 PLA is soluble in solvents like ethyl acetate or concentrated sodium hydroxide, so that might be worth trying if PVA doesn't work out.

Yeah, that's possible too, but a bit outside my chemical comfort zone.  Other similar options would be dissolving HIPS with limonene or ABS with acetone, although that's also getting outside my safe zone.

[neilestrick]

On 11/18/2021 at 10:42 AM, tkw954 said:

as long as the clay has started to mature enough to self support at that temperature.

Pyroplasticity could be an issue if you're firing the clay to vitrification. I'm also wondering if there will be issues with the expansion rates of the mold vs the clay.

If the form is too complex to be made with a multi-piece mold that can be taken apart, how is the slip going to be able to dry within the mold without cracking? If it's that complex it won't be able to pull away from the mold  and shrink without getting caught on the mold.

[tkw954]

38 minutes ago, neilestrick said:

Pyroplasticity could be an issue if you're firing the clay to vitrification. I'm also wondering if there will be issues with the expansion rates of the mold vs the clay.

I intended to just bisque fire at this step and hope that it would be self-supporting during the glaze firing.

 

34 minutes ago, neilestrick said:

If the form is too complex to be made with a multi-piece mold that can be taken apart, how is the slip going to be able to dry within the mold without cracking? If it's that complex it won't be able to pull away from the mold  and shrink without getting caught on the mold.

These are good questions.  I guess I'd want a body with low drying shrinkage.

[PeterH]

 @neilestrick's reply is a timely reminder that a clearer idea of what you want to make (preferably a picture) would give the experts here a better chance of anticipating problems.

BTW

* is the interior of of the finished article visible? Is a solid casting acceptable?

* what are your plans for surface treatment of the article (e.g. glazes)?

PS In brainstorming mode

I associate investment casting with pouring molten metal  to fill the mould. A cold ceramic analogy could be to use something like cement or a geo-polymer.

Shrinkage issues are much greater for clays, which also don't generate full strength until fully fired.

 

 

[neilestrick]

11 hours ago, tkw954 said:

I guess I'd want a body with low drying shrinkage.

You can only reduce shrinkage so much with a slip, or even with a moist clay for that matter. We deflocculate casting slips to reduce shrinkage, but it's still going to shrink a fair amount as all that water is evaporated away.