Saturday, April 23, 2011

Slip-casting the decoy-duck; step-by-step

There is probably nothing as rewarding as removing a slip-cast product from the mould for the first time. All your hard work, finishing off the prototype, and refining all the detail,  is revealed in the cast. It is always a magical experience, especially for students who are introduced to the process for the first time. The modeling of the prototype and the making of the moulds is a daunting task for most. But the rewards are manifold, especially when the process runs smoothly; when the split lines were done correctly, and one has established the procedure (sequence) for removing the various pieces of the mould, so as not to damage the fragile slip-cast product.
Preparing the mould
This is arguably the most important aspect of the slip-casting process. Ensuring the pieces are secured tightly. I normally make use of rubber tire inner tubes, cut into strips. However, for larger moulds they are often not strong enough, and for the first time today, I made use of large cable ties.  I joined a number of them together to be able to go round the mould. 
The slip can burst through the gaps (see image on left) where the various mould pieces join up. It is therefore critical that  all the pieces are secured properly and tight enough to hold shape when the excess slip is poured out. More about that further down the blog post.

Pouring in the slip – slip cast process.
Your moulds have to be dry. The water is being absorbed from the slip right up against the mould. The process takes about 15 – 20 minutes. Under normal circumstances, the slip build-up (thickness of the slip) should be about 10mm after the fifteen – twenty minutes. 
I make use of a timer as one inevitably forget. It also allows one to get on with other work whilst the process unfolds. When casting a fresh batch of mixed slip, it is necessary to test the build-up of slip in the pouring-spout. By blowing at the edge of the pouring spout, one can see the slip thickness (build-up) clearly. 
When using an old batch of slip make sure to sieve it first. This is necessary to remove dry bits of slip that fell into the bucket when emptying out the slip during the slip casting process (see image on the left). Once the desired thickness is achieved, in this case I left it longer - the moulds were not that dry yet and because the piece was lager than most products I have cast. The product is therefore slightly thicker.  

Pouring out the slip
Pouring the slip out can be a tricky business especially when the moulds are big and heavy and needs careful planning. I cast on a chair to be in a position to bend the mould over with a vessel on the floor, next to the chair, for the excess slip being poured out, to be poured into. Once the excess slip is poured out, it is much easier to pick the mould up and swirl it around as one waits for the slip to pour out. This process ensures that the slip is evenly poured out of the mould. One has to leave the mould standing upside down for all the slip to drain out of the mould. Leave the mould at an angle. Make use of a wooden stick placed on one side of the mould – to ensure the pouring-spout is raised from the floor.  It will stick to the surface and create problems when lifting the mould.
 Releasing the slip-cast product from the moulds.
After twenty minutes I turn the mould the right side up and cut the pouring spout out. I then remove the pouring-spout mould to speed-up the drying process. After an hour one can start the tricky process of removing the rather thick moulds from the fragile slip-cast product. 
Take care to work out which parts of the mould should be removed first, leaving the product to dry further until it is dry enough and strong enough to support itself and be removed in its entirety. The slip cast product starts to shrink and one must not leave it too long in the dry moulds. Often the first slip-cast product (from a new mould) can stick to the mould. This leads to the first cast being rejected. I was lucky this time, everything went smoothly.

Fettling the slip cast product.
Once the slip-cast product is dry and strong enough, one can start the fettling process, getting rid of the slip-cast seams caused by the gaps in the split-moulds. Only then can one begin to refine the rather fragile form and define the detail if and where necessary. Often edges break in the mould, when removing the solid prototype. As a result it becomes necessary to remove the excess slip build-up.

Slip dries quickly and working fast is necessary. I prefer to finish-off the product before it dries completely. Working on a dry slip-cast product is dangerous, the dust being a health and safety issue.
Further more, the product is very fragile when dry, and sponging the product at the leather-hard stage has the advantage therefore, of repairing any damage caused during the removal of the fragile slip-cast product from the mould.

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