Lessons learnt making a silicone mould from my face cast

 I finally managed to scrounge some hours over Christmas to make a silicon mould from my life-cast. Having tested out the approach on a hand (with bad results due to the naughty unstretchy blue silicone), I was waiting to decide how to make a mould box. In the end I just went to the local hardware store and bought the best match to the size I could find - which was an embarrassingly simple solution!

I did toy with making a fibreglass shell instead (a fibreglass 'mother mould') but chickened out because fibreglass is horrible stuff, and it felt like unknown territory. I'm sort of glad I did, but it did mean I had to use a tonne of plaster and probably a lot more silicone (BUT it meant I was confident that everything was always 'contained' in the mould box).

I followed the same approach as last time - mostly quite standard practice, expect the blue silicone which I will explain below. 

1. I filled the bottom of the box with a think layer of blue silicone to give me a nice level surface as the box was shaped at the bottom
2. I then vaccalined the silicone layer, put the cast in, and poured a second thin layer around it. This serves two purposes - it makes a nice seal around the edge, and it serves to 'lock-in' the position of the life-cast by creating a perfectly fitted 'depression' - i.e. I can remove it, and return it, confident that it is in exactly the same position as before by fitting it into the depression - which is important later (the downside would be that I'd loose a couple of millimetres off the bottom of the model, but actually, as fate would have it, I didn't, as we'll see later!)
3. I then put a layer of clingfilm over the life-cast, covered that with a thin layer of modelling clay, and then covered that in a layer of clingfilm
4. I modelled a 'bulge' on the top of the nose (the highest point as a I have a huge snozz) and the top of the chin (because I worried it might form an air pocket there) 
   
   
   
5. There was a LOT of space at the end of the tray around the chin, so I improvised and stood two yogurt pots to fill the space (filled with water so they wouldn't float up, obvs!)
6. I then filled the whole box with a fortunes-worth of plaster. As a bit of a miser, it broke my heart, but it was worth it! It took a LOT of plaster, and I had to mix it in batches, which was risky. Of course, the more full the box got, the less each load of plaster raised the level, as the face took up less and less volume
7. After a very long time, I felt confident to carefully remove the whole ensemble from the box and carefully remove the life-cast and modelling clay. I was VERY relieved to see that the clingfilm had held, as obviously a disaster scenario would be the plaster getting into the life-cast and bonding to it 
   
   
   
8. I now had a plaster 'mother mould' which fitted perfectly over the life-cast save for a thin gap. I left this to dry for about a week. I then patched up some rough areas caused by the clingfilm - I used a mixture of cotton wool and PVA, which worked, but took ages to dry - next time I think I should just use polyfiller!
9. The last step was to return the life-cast to the correct position in the mould, using the blue silicone layer cast around it as a guide to get it in exactly the right spot. I think put the mother mould over the top - slightly nerve-wracking as I had to drop it into place. Thankfully the plaster had expanded, so I needed have worried as it didn't fit all the way down anyhow. I then poured a huge amount of green (stretchy) silicone into the gap between the cast and the mother mould (the white spot in the middle is the tip of my nose showing through the silicone!)
   
   
   
10. Finally I removed the whole thing. I was relieved to retrieve the life-cast intact and not too badly covered in silicone (there seemed to be a small amount of cure inhibition due to the vaccaline perhaps). SUCCESS - a green silicone detail mould sitting perfectly in a plaster mother mould (if you are weirded out by the image, remember the face is concave!)
    
    
    

The whole process sounds fiddly (and it was) but it was also very time-consuming as each stage had to set/cure/dry before I could do the next...

I'm very happy with the way the mould came out. Unfortunately the green silicone bonded to the blue silicone layer cast around the life-cast - this was unfortunate as it makes it hard to reuse the mother mould in future (as I'll have to somehow recover the blue silicone to know where to place the life-cast to line-up with the mother mould), but great in that if forms a lovely stiff 'collar' around the mould that helps to support it and keep it in position.

To test it out, I cast some cheese-wax

Not too shabby at all! I'm very excited to start 'playing' with my new 'toy' ;)

Lessons Learnt

• The blue silicone base was a lovely surface to work from (smooth, frictive, level)
• Casting a thin layer of blue silicone around the life-cast was great for locating it in the box later, plus makes a nice 'collar' to the softer green silicone mould
• Patching the plaster mother mould with cotton wool/PVA was dumb, I should have just stepped up and found the polyfiller
• Letting the plaster dry thoroughly was smart
• Standing the two yogurt pots was a good hack, but a more systematic approach to filling unwanted free volume might save a lot of plaster next time
• The vaccaline seemed to kill a lot of the fine detail, and maybe caused some cure inhibition - if I'm going to do a major project again, I should just buy some real mould release agent
• Plaster EXPANDS as it sets - because crystals form. This is supposed to be 'slight' but was enough to mean that the plaster mother mould no longer fitted completely into the mould box
• I was better about trusting that the clay layer would leave a gap for the silicone this time, but actually the fact that the plaster expanded meant there was a bigger gap anyhow, as the box was slightly pyramidal, so the mother mould would no longer slide all the way to the bottom
• Having two vents was great - probably more would be even better as they 'cost' nothing except a small amount more silicone - pouring into one and seeing it appear in the other was very reassuring!
• I did a good job of dislodging air bubbles, but I should probably remember that as a result EVERYTHING gets covered in a layer of silicone - it made it's way up all the outsides of the mother mould, between the mother mould and the yogurt pots etc etc
• Marking up the mixing pot with a half-way mark and a full mark meant I could pour the silicone part A and part B directly into the mixing pot - risky, but MUCH faster than measuring in to more pots and then pouring into the mixing pot.