Adding toilet paper to plaster mold?

[joel_k]

Hi! I met someone who mentioned that they blend in shredded toilet paper into their plaster molds.

They start by painting a thin coating of pure plaster on the positive. (ie, just water and plaster powder, at a 70/100 ratio).

Once that is dried, they blend in some toilet paper into another batch of plaster (I am assuming she blends it into the water, then adds the plaster to slake/mix).

This mixture is thicker, and she sculpts it onto the positive to build up the mold. She said this makes the mold a lot lighter and easier to handle.

She didn't know the exact ratio of toilet paper / plaster / water, she said she did it by feel. 

I was wondering if anyone here had tried this, any tips for how to do it, and in particular what ratio of paper:plaster:water to target.

Thanks!

 

[PeterH]

I'd a faint memory of somebody trying this ...  got it.
Tutorial: Making paper plaster mould
http://andrea-ceramics.blogspot.com/2015/05/tutorial-making-paper-plaster-mould.html

I think I've also read of people just building up plaster layer by layer.

PS

With less plaster about it presumably will not be able to absorb as much water, so possibly slower casting and thick-walls/solid-casting an issue.

It also must have less strength, so high-volume moulds are probably out. Although I think I've read of somebody supporting the thin plaster shell in a bucket of sand.

 

[neilestrick]

It would take an awful lot of toilet paper to make the mold lighter. In Peter's link she's using paper pulp, which would be much easier to add in a volume that would actually make it lighter. I agree that it may affect how absorbent the mold is, but you'd have to do a comparison to see.

FYI the 7:10 ratio is for #1 pottery plaster. Other types of plaster require a different ratio.

[Jeff Longtin]

Welcome to the Forum Joel.

It sounds like you've been talking to someone who is describing making plaster molds for casting metal or wax. Is this person actually casting clay into their molds?

The process also sounds like the process for making "mother molds" for vertical sculptures. Is this person doing that?

The main reason we use plaster to make molds, for casting clay, is that it is porous and absorbs the water out of the clay. If we add something to the plaster then we're introducing air pockets into the plaster wall. In theory this could change the way the plaster absorbs water. Rather than add things to the plaster I prefer to simply make a thinner mold. The ideal thickness for a casting slip mold is about 1.5". (per USG) I've made molds that had thicknesses as low as .75" that still worked fine for casting porcelain. (They became saturated, more quickly, but they at least allowed for one casting a day.)

Would a thinner mold be an option for you?

My main hesitation with the method described is that the artist is creating a thin layer of plaster against the surface of the model. If she doesn't apply the next coat evenly she may be leaving behind large air pockets in the plaster wall. THOSE air pockets may affect absorption greatly. 

 

[Callie Beller Diesel]

Trudy Golley taught this technique something like 25 years ago. While you can slipcast into paper plaster moulds, as Jeff mentions it does wind up having uneven absorbtion because of the pulp distribution.  She used them primarily as slump/hump moulds for handbuilding, and they are indeed a lot lighter. Because you’re sculpting the mould, you can do things like put feet on them to make them easier to handle, and you don’t need cottle boards. Or have to worry about said cottle boards failing mid pour.

As I recall, mix both the skim coat and sculpted layer according to standard plaster ratios, and simply add the paper pulp (or she used cellulose attic insulation) that has been wetted and wrung out. She recommended the attic insulation because it had preservatives and fire retardants that hold up a lot better than toilet paper would. Newsprint pulp does work, but you have to dry the resulting mould thoroughly. 

Note: you do NOT want to use the fibreglass blow in insulation. It has to be the cellulose kind.