This Instructable will not cover how to use or fire Precious Metal Clay, rather how to create and test your moulds to ensure the desired effect is reached before destroying expensive clay!
As with everything, have your work station set up neatly with all items you will require to make the process easy.
Spend the time getting to know every tool and chemical, and rehearse the order beforehand. Also, when creating the two halves, you want them to overlap and have some quirky ridges so the halves won't slide around when you are trying to mould the precious metal clay.
Step 1: Materials and Problem Solving
2 part quick and cold temp setting mould compound (such as silicon or rubber)
Organic Object from which to create mould
Ounce/Gram Scales (if you wish to be precise)
Plastic Wrap (Saran or Glad Wrap)
Clay, plasticine, etc (children's clay/dough of any sort that results in a smooth surface will suffice)
Ruler or flat hard object
Smooth working surface
A. Read your compound instructions carefully and don't throw them away like I did! There will be a set "mixing time" and then the compound will start setting - be very aware of this and do a few test balls first while watching the second hand on a clock to know how the compound works.
B. Measure out each part of the mould compound exactly according to the required ratio using the scales.
C. Warm each part of the mould compound separately by working in hand until malleable.
At first I was having trouble as the mould compound was setting looking like a kitchen sponge, which made my mould rough and lost a lot of detail. The sponge look can result for a few reasons
a) the two parts are NOT very close to required ratio amount (weigh them!)
b) OR your mould compound is too old and has gone off (who would have thought rubber can have a shelf life!)
c) OR You are mixing the two parts for too long and the compound is trying to set
d) OR each part is not at its optimal temperature for use
e) or a mixture of all 4 reasons.
Mine was a mix of reasons a, c and e so after some more test runs I figured out that I couldn't mix past 20 seconds, had to warm up the separate parts for a while and "eyeballing" the amounts didn't work for me.
Step 2: Create Single Layer Mould
A. After your mould compound is mixed, lay it out into a shape that is deeper and wider than your object on a plastic mat or smooth table top.
B. Carefully press the found object HALF WAY INTO THE MOULD. Using a flat plastic ruler can help press the object evenly into the moulding compound. You have one chance to get it right and cannot fiddle with the object while the mould is setting so resist the urge to lift the object out right away.
C. Let mould set. Lightly press a finger nail into the base of the mould to test the setting process. The mould is set when the fingernail mark no longer stays in the mould. After the mould is set you can take out the found object - be careful so that it doesn't break making the second half of casting difficult.
D. For a single layer mould, skip to Step 5.
Step 3: Prepare Bottom Half for Top Half of Mould
B. Carefully put the found object back into the bottom half of the mould until it sits snugly in place.
Step 4: Create Top Half of Two Sided Mould
B. Mix more compound together than for the bottom layer and roll into a shape larger than the bottom layer.
C. Press top half down onto object and bottom layer of mould so that found object is completely covered and top layer overlaps three sides of the bottom layer. Leave mould to set.
The top half of the mould should lock down onto the bottom layer. I positioned the top half so that it hung over the bottom layer on three edges. This makes it easier to line up the two halves when doing the actual casting of clay. See photos on bottom.
If the two halves do not line up easily, recast the top half of the mould.
Step 5: Test the Mould
B. Roll snake (or ball depending on shape of moulded object) of test clay a little bigger than original found object. Place into half of mould.
C. Press the two halves firmly together until you have a snug fit. Gently separate the two layers of the mould. Watch closely to make sure the clay inside stays in one half of the mould - and doesn't split between the two layers! This should not happen if you have used enough olive oil.
D. Carefully lift out the test clay from the mould for inspection.
If the mould isn't quite right, try repeating step 3 again.
Testing the mould with a cheap clay, plastacine, polymer clay or even playdough (as I used here) lets you know if you are happy with the mould detail of the found object without wasting precious metal clay. It will also give you an idea of the volume of PMC required with minimal wastage.
You can see that I did not coat the mould with olive oil before squishing the test clay. Your clay can stick to the mould if not coated, ruining the casting.