Making a mould of Student Work-JBL

[JBL]

I have a student who has created a highly detailed relief sculpture using low fire clay. It’s about 11” x 17”. She’s done a great job keeping it well wrapped while working on it. I think she is anxious it might break during the bisque firing. I told her to minimize the risk of it breaking we would:

-let it dry slowly, unwrapping and then rewrapping daily to slowly allow exchange of air

-we can put wax on any protruding delicate areas to promote even drying 

-candling after bone dry and slow ramp up during bisque firing

-silica sand on kiln shelf to reduce drag as work shrinks

-if it does break during bisque (hopefully not) I told her we can mend with Bisque Fix and refire

-even if it breaks with glaze firing we can glue or she could do kintsugi (her piece is an Asian inspired relief sculpture so it would look nice) 

Despite all reassurance she is asking about creating a mold of her clay sculpture as it is currently leather hard, prior to bisque firing, just in case it were to break then she could cast it in plaster. I have experience with dental alginate but never with low fire clay you are hoping to fire. I’m worried her sculpture might get damaged just in the process of making the mold (? undercuts). Also, I know she will loose some fine detail. I know alginate will frequently trap air pockets and then she would have to fix those at some point. She stated if the clay was damaged at this leather hard stage due to the mold making she could repair it. 

My question is if it’s worth all the trouble making a frame, purchasing $50 of alginate (if even that’s the best product), soaping it with a release (which I don’t know if that will impact the clay, firing and glazing?) just because she is fearful it might break? is alginate even the right product? Will soaping the clay create problems with drying, firing or glazing? To me it sounds like an expensive and timely  process just to avoid the “possibility” of it breaking, which given the steps I mentioned we would likely avoid. Any advice is most appreciated. 

[Mark C.]

Not worth it 

It will mess with the details as well before firing

fire really slowly after a good lomng drying time and you should be fine

[JBL]

Thank you for your feedback

[neilestrick]

You'd want to make the mold from plaster, but making a plaster mold of it will likely ruin it, and any problem you have with firing the original will also be a problem with the cast ones. I think your plan for dealing with the original is good, though, so you should be okay.

[JBL]

Thanks for your advice

[Jeff Longtin]

Hello JBL,

Molding unfired clay pieces is challenging. Alginate probably doesn't need a separating compound. I've poured it against skin and it pulls off easily. I would test it to be sure as this is a precious piece.

I've never used alginate against an unfired clay surface but one advantage, over plaster, is that it doesn't heat up and may not cause the clay form to soften. (Plaster heats up, which generates steam, and often cause unfired clay pieces to deform before the plaster sets.)

One thing you could consider is to fire it to cone 012 or 010 just to transform the piece from clay to ceramic. (I forget the exact temp?) Its still a fragile piece but its no longer clay so you have a few more options. I did this years ago to minimize the impact of shrinkage and then spray painted the piece before I poured a plaster mold.  (To seal it.) It held the paint fine and made the mold making process much simpler.

If you want to send a picture I'd be happy to offer suggestions once I know what the piece actually looks like.

[JBL]

Thank you. I’m going to have a closer look at it tomorrow to see if there are undercuts. I’ll ask the student if she’s ok if I take a photo. Great to have all the advice  

[JBL]

I had a close look at the piece yesterday and the undercuts are not bad at all. Plus it is a bit smaller than I thought so will need less alginate. If it will make the student more at ease we will do a mold for her as a back up in case the work breaks. Thank you for all the feedback!  

[Hyn Patty]

I'd go ahead and fire it as it is - breaks can be fixed post firing as you mentioned.  If she still wants a mold made of it then she could use a brush on silicone rubber like Mold Max Stroke, make a plaster mother mold shell around it, then cast her back up in other non-clay materials like plaster or resin.   This is called a waste mold and isn't intended to be used for lots of production casting.  If she wanted to cast a lot of resins or plasters then she'd want a rubber block mold.

She could then cast a thin hollow rubber version using Mold Max Stroke, filled with a plaster core after the rubber sets, from which she can then practice making plaster molds for slip casting.   Fill with resin instead of plaster if it's particularly fragile with thin parts.  Making the rubber thin and supporting it with plaster (either core or mother shell depending on whether it's the mold negative, or the master positive,) would save her a ton of money on casting rubber which isn't cheap.  The smaller sample bottles available from places like Smooth-On are ideal for projects like this if her piece isn't very big.

Edited March 1, 2023 by Hyn Patty
Added data