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.

Slip casting the Rats

Slip Casting. One assembled rat and another drying supported in the mould.
I have started slip casting the rats (to be assembled by hand to create variable stances - an animated approach) to develop appropriate surface options for the installations. I hope to do pairs and various groupings exploring different themes. The first set of pairs will be decorated with decals/transfers, and therefore will remain plain (no under glaze colours and or colour glazes will be used) only a transparent glaze - rendering the rats an off white colour (cream).


















Pair of slip cast and assembled rats.

Mould making for slip casting the rats

Mould making done simply, efficiently and effectively. The First phase of mould making. Splitting the prototype, taking care of undercuts . Building the divisions (Leather-hard clay slabs) - determined by the number of sections to be moulded. I normally secure the prototype in a clay slab to ensure no plaster penetrates below the prototype and it also provides support to the extruded plastic sheeting (to hold its shape - determined by the thickness of the plaster walls).





















Preparing the mould casings - using extruded plastic sheeting and the ever clever cable-ties to hold the casing together; ensuring no plaster escapes the casing. Soft soap is applied and left to dry to ensure that the moulds seperate from each other.





















Poring spouts are attached before casting the final pieces normally the bottom that contains the opening (pouring spout) for slip casting.




















The moulding process is complete - the prototypes removed and moulds refined before drying. The keys that holds the pieces together are done with a large coin held at an angle and turned scraping the negative circular form as seen in the above image.





















The completed moulds and prototypes - ready for drying. The tail was cast during the previous moulding process and ready for slip casting.

Rat-a-tat-tat-tat-tat-tat-tat-tat-tat

Got you. Prototype for installation - see post entry below.
Modeling almost complete. I always review the form and shape the next day, before the moulding process begins.

Showcasing modeling of prototypes.

Academics need holidays to get down to serious work in their studios. That's exactly what I am doing - two weeks dedicated to remodeling and moulding rats for slip casting an installation of animated rats. The rats are designed to form a carpet - their tails reminiscent of tassels when they are placed in Escher type formation (opposite directions). The majority of rats will lie motionless, whilst their radical counterparts, will be roaming off in opposite directions creating havoc (reminiscent of the black plaque). Decals will be created (fired on the glazed bodies of the rats) to add meaning, based on either tattoos and or carpet designs depending on the theme. The rats represent a plague of intolerance spearheaded by extremists and fundamentalists.





















The original prototype from which five plaster moulds were made. In the background are a few slip cast rats experimenting with surface development options.
















The process begins - remodeling the prototype into various sections. The various body parts and feet (to be modeled) will be moulded separately to be in a position to animate the rat; assemble various options creating applied movement in the installation.





















Coffee break; reflecting on the shapes and interface between body and head (variable options once slip cast and assembled).


















Refining the integration of the body and head. The modeling clay is called Y2 - clay. I use a 100 watt spot on the prototype to keep the clay soft for modeling and another lamp close to a batch of clay from which I take pieces to add to the prototype. The clay goes very hard once the heat is removed (light switched off). For those who are not familiar with the clay - it is fantastic and makes the moulding process that much easier. You never have to keep the prototype wet.

















Comparing the integration of modeling with the original slip cast, fired and decorated product.






















Considering various animated options before refining the prototype and adding the front feet (to be modeled and cast separately)