Making plaster molds without cottle boards

[AaronRotchadl]

Anyone have any experience making plaster molds without cottle boards? I want to make some plaster molds of desert towers that are roughly 8"x8"x14".(see photo) I've made one sculpture/mold so far using the method from Andrew Martin's book where the cottle boards are placed with the sculpture upright and then the seam lines are formed with clay by reaching down into the mold.(see photo) The experience was nightmare-ish in the moment and the mold, albeit ugly, works like a charm for slip casting, but is soooo heavy(like 60 lbs.) because of the way the sculpture tapers. So in order to save on weight and $, I'm looking into making the molds by doing a "hand lay up" style plaster mold like the one in this video, where I make walls out of clay at the seam lines and just brush/spatula the paster on. This seems like an OK idea in my head but I'm wondering how difficult it is to achieve an adequate 2" of plaster all over the sculpture? Should I mix just one estimated batch for each "piece"? Since it will probably be slip cast, how will I keep it square so it will stand on its "top"? I'm also pretty open to doing a slab/press mold where the mold thickness and needing to prop it upright won't be an issue. Any input, experience, and/or advice would be much appreciated. Thanks in advance!

 

[AaronRotchadl]

sculpture photo

 

[liambesaw]

Why don't you want to use cottle boards?

Instead of trying to put your draft line on it while it's in the boards, lay the piece down so it's 16 wide by 8 tall and 8 deep, (horizontal) and prop it up so it seems fairly level, then use a miter square to find the draft line by moving it around the horizontal object and Mark it with a pencil.  Then you can build up clay to that point or make a Masonite draft board and seal around it with clay and attach cottles.  Then simply pour, flip, remove blockout, soap, pour.  

I just had to do this for an object that is 6x6x13.  It was a big pain but I'm glad I did it, have cast it 5 times now with great success.

 

Edit: the problem with brushing plaster on like in bust blanks, is that the plaster doesn't stick to itself reliably.  In a bust mold, it is used once and then chipped off, so it is actually beneficial to have a mold that isn't durable.  In slip casting you want a durable mold that lasts so you can use it over and over.

[Mark C.]

For one time mold the brush and goop thing will work but for real molds the cottle boards make 10000% more sense. Well noted in above post.

Plaster is cheap and so is wood for making a barrier to hold it.Scrap wood is free.

[AaronRotchadl]

Hey guys- Thanks a lot for weighing in! I'm going to take y'alls word that cottle boards are the way to go. I think I may try to make the "base" its own mold and attach it to the tower in order to keep the weight down and use less plaster. 

[neilestrick]

The method in the video would not be good for slip casting. You need a mold that is thicker, and that can be set upright during casting and set upside down for draining- a nice rectangular block. If you plan to press mold, then you could use the method above, but even then I would make it a lot thicker than they did, and imbed burlap sheet in the plaster to add strength.