The idea of a bronze sponge came from my list of domestic objects and their primary functions, I considered the sponge as an overlooked yet important everyday object who’s function was to absorb. So in order to subvert and remove that function the object needed to become rigid and heavy. The process of casting the sponge was a tricky one as by its primary function the sponge wants to absorb liquid, and almost every casting material involves water. I tried making the sponge no porous by soaking it in varnish, painting it with PVA glue but ultimately the sponge was too absorbent and my efforts were futile. However after talking to Mark he suggested that I try casting the sponge in alginate, the stuff they make moulds for gum shields out of, because it had a very fast setting time and would probably not be too thick to be absorbed by the sponge.
The next question was what kind of sponge, I have already discounted natural sponge which would have been easier to do as it could be a direct burn out. I had been working with car sponges which have that figure of eight shape that is a symbol for a sponge however there was not much viable texture to the sponge, as the texture it did have was very small and to immediately apparent. I found that cellulose sponges had a better texture but they were rectangular not linking to the generic shape of the sponge. I researched quite a bit and found their was a hybrid of the too, it was a little smaller than I initially wanted but in hindsight it is actually quite relevant as it fits perfectly into the grip of your hand giving the object a relation to the body.
The casting process was relatively straight forward the sponge needed to be in two parts so that when the bronze was poured their was less chance of the piece warping out of shape. So to do this I made a one part mould of the sponge with the intention of making filling it up to the top and waiting for the sides to harden before pouring out the excess for one far and just filling the bottom of the mould up for the top. This meant that the top and bottom of the sponge were effectively the same. The only problem with alginate is that the mould begins to decay after just one use so I could only get three whole sponges out of the mould. So I made two into the wax tree and kept one back encase they did not pour, so i would not have to start again from scratch.
Spuring up the wax, was relatively straight forward but required a bit of planning each piece had to be staggered and ideally running at a parallel to the centre of the tree, which was a hollow wax cup with a two inch thick stick of wax, welded onto it. From each item there needed to be three sprews. One going from the bottom of the centre stick, one from the base of the item leading back onto the centre stick and final another one leading off the first attached to the outer rim of the cup, this was to allow air to get out once the bronze was poured. What I had in wax here was a positive representation of what was going to be in bronze, in not a little larger if you account for the shrink rate of around 4%.
The next element of this laborious and time consuming process was to dip the wax tree into the slurry and coat in fine and then corse layers of ceramic shell, as the coats went on the tree became increasingly heavier. Their was four layers of fine ceramic shell and four layers of corse which were coated on the tree at intervals of at least four hours. They were left to dry over the weekend and the the wax was burnt out in the kiln, leaving a day in between the pour to make any repairs to the shell.
After a false start, the bronze was poured into the shell and left to cool, after about an hour I could crack away the shell and see if it had poured. They poured really well, but that was not the end of the hard work. Each part of the sponge had to be cut off from the tree using a grinder and then linished so that it was completely flat. This was when it became apparent that the pieces had warped slightly as they no longer fit together perfectly. However, weld could be built up where the gap was a little to big and then linished back to finish. After I had got the two pieces TIG welded together and linished so the weld was not visible they needed to be grip blasted to get out the remainder of the ceramic shell. Next step patternation.