| MOLDS | SLIP | TIPS | MY WORK |
Molds are time consuming to make typically 2 hours or more should be set aside for this project. Potters plaster should be used for molds since it is stronger. Once the mold has been made, with careful handling, it can survive about 100 uses before the chemicals in the slip eat into the surface.
The object to be made into a mold should be strong enough to be sunk half way into a block of clay. Walls should be erected leaving an inch gap around the object. By putting several lumps of clay on the base block you will make a key joint for keeping the two halves of the mold central. Fill with plaster and hope it doesn't leak past the walls.
After drying overnight you can turn the mold over and remove the block of clay. Loosen the object without breaking the fragile mold edges then paint watery clay over the surface and erect the walls again. This clay barrier stops the plaster surfaces sticking to each other. Fill with plaster again and leave to dry. Carefully break the two halves apart and remove the original object.
It is important to choose a symmetrical object or one which can be removed from a mold without having to break it. Some complicated molds require 3 or more pieces in order to make the shape. When you have slip cast an object it invariably requires some work from trimming to straightening out. Remember firing and glazing will cover minor blemishes.
Bowls can be cast from 1 mold by pouring plaster around the bottom and humps can be made by filling up the inside (making sure it wont stick. Anything could be used for a hump from an empty Chinese plastic dish to a mixing bowl. A hump can be draped with rolled clay which can then be left to dry. A foot-ring can also be attached sine the new slab bowl is upsidown.
Powder and crush 5kg of clay to a powder.
Add 7.5g of sodium silicate and 10g of soda ash to a small cup of warm water. These are chemical deffloculants that chemically change the clay particles allowing them to float in suspension in far less water than they would normally.
You need to aim for a slip of 1750g of clay per litre. This works out at 2.8 litres of water and a major point should be made here that the thicker you can make the slip and still keep it runny the better.
I would put 2 litres of water in a bucket and most of the deffloculant then add the powdered clay a handful at a time. You should aim to create a thick syrup and to this end you should have the remaining litre of water on hand and also add small drops of the deffloculant. Be careful if you get the mix wrong the clay will congeal in lumps and you will have to add more of everything until you restore this rather delicate balance.
Once you have a thick syrup you need to sieve it into another bucket through an 80 - 100 mesh sieve. This is the good bit, the sieved slip will land in a lumpy thixotropic gel but - voila - give it a stir and it turns to liquid again. The sieved slip will lose moisture to the atmosphere so can be topped up carefully with water. Slip once made can last several months but MUST be sieved each time you want to do a casting session.
Store the slip in an airtight bucket, you can use bigger sieves if you want to speed subsequent uses but give a good sieve at least once a week if using regularly. Molds should be stored with the openings sideways to avoids getting contaminants inside. Rubber bands can be used to hold the halves together. Rough edges and crumbly bits should be removed or skimmed off. Slip can be coloured with oxides in very small proportions but obviously once you do this the slip cannot be reverted.
My attempts
I made a cast of a bear using a statue about 4 inches tall. The walls were bits of badly cut hardboard and the gaps were plugged with clay. Needless to say it was a bit of a bodge. The plaster leaked out and I ended up using hands and feet to hold everything together for the 10 minutes or so that it took for the plaster to harden enough to stop pissing out of the gaps. You've got to laugh, it was quite funny but don't do it on the kitchen floor it really makes a mess. Mixing plaster is also a bit of a hit and miss thing, you have to start with enough water to fill the mold and with one hand add handfuls of plaster while stirring it by hand with other. The plaster starts getting stiff turning to double cream and then it must be poured into the mould. The bucket needs to be cleaned each time so avoid getting lumps down the kitchen sink else you will block it up. Slip poured in a mold should be left 10-30 mins depending on the thickness of the walls you require before pouring it back into your slip bucket in one smooth motion (didn't tap it). The mold should be left 1-2 hours before gently breaking the seal and removing you ware. be careful dry slip casts are very brittle, some people have likened it to a popadom.