Welcome to Part II. Where was I? Oh yes, on to moulding.
If this was a bronze, I would need to do a two piece mould so that a hollow wax could be painted into each side for lost wax casting. However as this is for a resin reproduction, the more joints or seams I have, the more of a nuisance and a disfigurement it will be - so I am going for a single skin silicon mould with minimal cut lines. In this case, just one from base to under the chin. I will make the final cut in the mould with a scalpel. Oops! I nearly forgot the registration points for the mother-mould.
As you can see, I've incorporated a seam divider under the chin in order to give me a guide when cutting mould at the end. The rod coming down from the chin will also act as an air release in the mould to avoid air bubbles becoming trapped.
If your model is made with a soft clay or is very intricate you need to make a "block mould" before going to the next stage.If you do not you could damage your original beyond reasonable repair.
A block mould would be the same process up to now except at this point you would build a containment around your clay original and simply fill it full of vacuumed silicon rubber. You will get a perfect reproduction of your original but made with a lot of expensive silicon. You will then need to use a hard copy reproduction to go on to the next stage. Of course if you only want one copy for yourself you can stop here.
As I am making a "skin mould", next I need to protect the my original clay sculpture, I use kitchen foil but you could use cling-film just as well. I like foil because there will be some heat generated in forming the mother-mould and I think the foil helps to reduce the effect on the surface.
I am now going to encase my protected original, in a skin of wet clay in order to form the mother-mould. I use wet clay because it is soft and forms over the original without the need to exert any risky pressure which might cause damage or movement.
I roll out the clay (like baking a cake) with depth guides to maintain an even thickness and then lay it over the model until I have fully encased the original
The pipe was intend as the main feed for pouring the silicon but in the event, I used a different feed. However it was not wasted as it formed a reference for the silicon and a riser for any trapped air.
There are many ways to form the boxing for the mother-mould and there are also a few alternative materials to use for the purpose. This is just the way I did it for this figure.
In order to form two halves, I often use, as in this case, plaster board. It is strong and easy to cut with a jig-saw. Firstly mark out your work as accurately as you can and then cut it out.
This is a little bit simplified but now the plaster board needs to be fitted around the encased original and box shuttering put in place to form the plaster.At this stage, you have to hope your armature was strong enough for it not to collapse, I usually support it underneath to help avoid that situation.
Any gaps around your work piece need to be filled bearing in mind that you need to avoid undercuts or any awkward shapes that might make casting difficult. Also because I've used clay and it will stick to the plaster former I give it a liberal coating of release agent made up of petroleum jelly and white spirit. When I get around to taking the plaster back-up's off later I don't want anything to catch and risk damaging the original.
So now I'm ready to lay on the plaster . . . . . .
Hope you can stay with me for Part III
Thanks for looking