Plaster help needed

[Fank]

Hello sculptors!

I need help learning a bit more about the potters plastering I'm using. I am producing a large solid form from a recent silicone mould I made last week. I cast a positive from it using potters plaster 3 days ago and there is a large crack today. I think perhaps the direct sun and hot room might be the cause for the fast shrinkage. While pouring the plaster, I inserted a cobbled support in the center for mounting later (made of wood and nails. Hopefully, this isn't a contributing cause of the crack. At this point, is it safe or even possible to add fresh plaster to repair the surface of the crack? Will this piece split at some point?  

 

Edited August 8, 2023 by Fank
added photo

[JohnnyK]

Welcome to the forum, Fank...There's a good possibility that the added heat may have caused the crack, but if you are intent on saving the piece, what you might do is drill the piece perpendicular to the crack and counterbore the holes at each end to accept a threaded rod to which you could attach washers and nuts to hold the piece together. Then you could plaster over the holes and do whatever carving would be necessary to finish the areas. Then you would open the cracks to accept the plaster for repairs and then do whatever carving that would be necessary to finish the piece. That's how I would go about it if I wanted to save the piece. The alternative would be to cast a new piece.

[Jeff Longtin]

That's a pretty deep crack.  Hard to say exactly how the plaster will react in the future but it certainly doesn't look good. The crack appears to undermine the wood plate so I'd be hesitant to use it to anchor a bracket.

Back in the 90's I made molds for a gift shop that made and sold cement figurines. They also cast table bases made of cement. They would roto-cast those. Previously I'd never heard of roto-casting. There are certain cements made for casting figurines/sculptures. Have you tried any of those?

All of which is to say I would recommend you not solid cast this/these. Mix a small amount of plaster/cement and brush it on the inside of the piece. After 5 - 10 minutes apply another coat. Maybe do a third coat. Then, after it completely dries, you could GLUE a bracket into the cavity. Lots of epoxies are super strong and with a hollow cast you have less weight to worry about.

Good luck.

[Clay Wallower]

I was using maple pieces as my masters to make molds.  I poured a plaster mold and let it sit overnight to harden and dry.  In the morning the wood had absorbed enough water to swell and split off a side of the mold.  I decided that I could not find the perfect balance between a wooden master that sits a long time (swells and breaks the mold) and pulled off the mold early (damaging the soft plaster).

The way that crack runs I would be suspicious that the wood absorbed the water in the plaster after it cured and swelled (5%-10%) -- pop.

The current technique I'm trying is HDPE masters.