So, the time has come for us Black & White Casting folks (erm... Toby and I) to push the envelope in our search for more detail in our casting work. So far, we've been the proverbial* sand-men, packing our formative objects de wax within a silica tomb before applying a generous dollop of hot metal. However, it can be advantageous in some instances, particularly with work where a high degree of surface texture is required, to use an alternative method - namely ceramic shell casting.
In theory, this is an easy process, you simply make up your wax pattern (in my case a model of a hand), coat it in several layers of slurry, letting each layer dry before applying the next, fire the whole thing (to burn out the wax and to harden the ceramic shell), then tip in your molten metal.
The trick is getting the slurry mix right.
You can buy slurry mix off the shelf (albeit a shelf in a specialist casting shop), but it costs a fortune and we Black & White Casting types "prefer" (ahem) to bit somewhat thrifty when it comes to materials.
Thankfully, Toby had come across a "secret formula" on one of his more recent casting courses, so we used this as our basis for testing.
My first test begain with the creation of a small wax keyring type object. I then mixed up the soon-to-be-not-very-secret secret formula, which consisted of equal parts of plaster, sand and vermiculite (the latter being old-fashioned loft insulation). It was a right pain to grind down the lumpy vermiculite into powder, but I sort of managed it and pressed on regardless, adding water to the mix to make the aforementioned (and very aptly named) slurry.
The slurry was the dribbled (for want of a more arty expression) over the wax keyring and put aside to dry. Once dry (ish) I added a further coat and repeated this process a third time before deciding enough was enough.
I promptly took a couple of months off over Christmas, eat and drank a lot and avoided all things arty (I'm lazy like that).
Having had a long time to dry (in my kitchen) the slurried-keyring had gone rock hard (if a little powdery), so with the dawning of the New Year and a trip over to darkest Wiltshire, the time was nigh to melt out the wax, fire the ceramic shell and get that metal poured.
In short... success. The ceramic shell survived half an hour with of heat blasting in our bespoke melt out oven/ceramic firing kiln (aka a heat gun and Twinings Tea tin) and didn't even collapse/explode when we poured the molten ali in! Additionally, the surface of the final keyring was pretty good considering I wasn't THAT careful in it's construction and I hadn't paid too much attention to having a pour cup (to allow a head of metal to press down into the mould) or sprues or anything any sensible caster would have done.
The next step was to refine the process somewhat. With that in mind I made a better test item (molten wax poured into a metal tin to produce a uniform and smooth shape), modifed with letters stamped into it (for extra detail). I also purchased high quality vermiculite grinding apparatus (a £12 blender from the Argos sale) and chopped that lumpy crap into proper powder. Mixing it with sand, plaster and water produced a much smoother slurry, which acted a lot like that special stuff that flows like a liquid, but is hard when you punch it (look it up on t'Internet if you don't know what I mean).
My final act before close of play was to slurry-ify (technical term) the new wax pattern mentioned above. With luck, when we return to "the Shack" I will be greeted with a strong, solid ceramic shell which I can continue to build up and (eventually) pour molten metal into!
See you next time.
* To be fair I don't actually know of any sand-men related proverbs, so you'll just have to bear with me I'm afraid.