Ceramic Slip Moulds - A Beginners Guide to Pouring

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Ceramic Slip Moulds - A Beginners Guide to Pouring

I get quite a lot of emails from beginners asking me what to do with slip moulds so here's a beginners guide. It's not meant to be definitive, just a simple guide for people interested in taking up pouring as a hobby.

Equipment you will need:
  1. A kiln - or at least access to someone elses kiln
  2. Slip (liquid clay)
  3. A jug or pouring table
  4. Moulds
  5. A soft brush to brush out the moulds before pouring
  6. Bands or straps to hold the mould together while pouring
  7. Tools to clean (fettle) the cast
1. A Kiln
This is the most expensive piece of equipment you will need. For beginners I would recommend that you see if you can get access to someone elses  first - maybe at evening classes or a friends. A small new hobby kiln can cost several hundreds of pounds & even though they are often for sale second hand, they are heavy, & awkward to move any distance. It's better to use someone elses until you are sure it's a hobby you want to follow. If you do buy a kiln make sure it's in good condition, that all the elements are working & the kiln brick lining isn't damaged. Try to get one with a controller that you can set or you will have to use 'cones' to check the temperature & watch it almost constantly. Buying a kiln is outside the area of this guide but I'd be happy to give any advice to anyone who contacts me.
Search for a kiln on ebay

2. Slip
Slip is liquid clay. It's not just clay & water though, it has several other things added to make it the right consistancy to pour into moulds without having too much water in it. Too much water in a slip will wear your moulds out very quickly. Slip is available from most hobby ceramics suppliers & I sometimes have it for collection only.

3. A jug or  pouring table
You will need a jug to pour slip into the moulds. Just about any jug you have at home will do - I used to use a 2 pint plastic measuring jug. If you get really 'into' pouring you can buy a pouring table that has a pump to pour the slip into the moulds but these can be expensive so it's best to start out using jugs.

4. Moulds
These are the things that will make your finished cast - a dog, cat, plate, mug or whatever else you choose to make. They are made from Plaster of Paris & are specifically made for use with slip. They do NOT work with other things like Herculite, chocolate, sugar icing or cement. If you pour Herculite or plaster of paris into a slip mould you will not be able to open the mould as it will seal together - it's only use then would be as a rather unattractive doorstop.
Search for slip moulds on ebay

5. A soft brush
When moulds are stored they gather dust & before pouring each mould it should be gently brushed out to remove any dust or slip spots. A soft artist paint brush is fine for this job.

6. Bands or Straps
Most slip moulds have at least 2 pieces that need to be held together while the mould is poured. Large 'rubber bands' or mould straps are used to do this. You can buy moulds bands from a hobby ceramic suppliers but most people use car inner tubes cut down. Straps are used to hold larger moulds together as they are stronger & don't 'give' as much, they are available from hobby ceramic suppliers.
It is essential to have enough bands or straps to hold the mould pieces together, if too few are used the weight of the slip will force the mould open & slip will pour out - usually onto your feet!

7. Tools to fettle the cast
Once the cast has been removed from the mould it will need 'fettling'. This is completely cleaning off any seam lines where the pieces of the mould have met & restoring any detail that has been lost.  There are dozens of different types of pottery tools available, some people prefer plastic tools whilst others prefer metal & only trial & error will tell you the best for yourself.


How To Pour a Ceramic Slip Mould

Gather all the equipment together that you're going to use & work in a clean area. As this is quite a messy hobby this may sound silly but if other things get into the slip & contaminate it, the cast could break when it's fired in the kiln.

Decide which mould/s you're going to pour, open them up & brush them out with a soft brush so that they are clean. Put the pieces of the mould back together - most moulds have 'keys' or natches to make sure they are aligned properly. Fasten the pieces firmly together using either bands or straps. The bigger the mould the more bands & straps it will take to hold it together. It's better to use too many fasteners than too few because if you don't use enough, when you pour in the slip its weight will force the mould pieces apart, ruining the cast, wasting the slip & usually covering your feet in slip.

Stand the mould on a flat surface with the 'spare' upwards. The spare is the hole where slip is poured into the mould. Some moulds have 2 spares, the second one is usually an addon - eg. a leg or arm, that will have to be poured after the main part of the mould if it is on a different side to the main spare. Pour slip slowly & evenly into the mould through the spare. Try not to pour slip directly onto the pour face, pour it against the sides of the spare if at all possible. Fill the mould right up to the top of the spare & leave it for a few minutes. You will notice that slip level has dropped - top it back up.

How long to leave slip in a mould is a bit like asking 'How long is a piece of string?' There are many things that will affect the time needed, how thick do YOU want the cast, which slip are you using, is the mould completely dry, how big is the mould & even what the weather is like. The plaster which the mould is made from will 'pull' some of the water out of the slip leaving a thin cast against the inside of the mould. The longer all the slip is left in the mould, the thicker the cast will be. Different slips do this at different rates. If the mould was damp to start with it will take longer to absorb water from the slip. If we're having  very damp weather (normal in England) then it will also take longer unless you're lucky enough to be working in a heated environment & not the garden shed.

A rough guide to the thickness of the cast can be made by checking the thickness of the cast in the spare. If you think it's thick enough then it's time to empty the mould. Slowly & gradually turn the mould over so that the spare is on the bottom. Don't put the slip from the mould back in with new unused slip, put it into a separate bucket for use again later. The mould must be emptied gradually, if you just turn it over the slip may pour out too quickly & form an airlock which will either leave most of the slip in the mould or will pull the cast away from the pour face.

Once the mould is completely empty turn it back so that the spare is uppermost . The spare can now be carefully removed although there are some moulds where this isn't possible. Leave the mould with the cast inside until the cast is hard enough to support itself but not so hard that it's cracking. Plain moulds like mugs or plates are fairly easy to judge but if a mould has a lot of detail it needs to be watched. If it's left in the mould too long any undercuts or details may cause the cast to crack. This is because the cast will be shrinking slightly & if it hasn't room to shrink, it will crack.

When it's firm enough to support itself carefully open the mould. Make sure you lift each piece straight off & not at an angle or you may destroy the detail. If a mould won't open, leave it a little longer. If it still won't open try gently tapping it all over with a rubber mallet. Once open, carefully remove the cast & leave it until it's leather hard - literally like you would expect  fairy thick leather.

It now needs 'fettling'. You will notice on the cast that where the pieces of mould met there is a line. This line is removed using whichever tools you favour. It needs to be completely removed as if any is left it will raise up when the piece is fired. Try to restore any small detail that may have been lost by fettling & use a sponge to level out the spare hole (which is usually underneath a cast).

Now leave your cast to dry out completely. It MUST be totally dry before you fire it in the kiln or as the kiln heats up so will the moisture in the cast & blow it apart. Most slip casts are fired to around cone 04 which is 1049 - 1060 degrees depending on which make of cone is used. Most kilns have safety features that prevent them being opened when they are firing but if yours hasn't got one of these please don't be tempted to just open the door a bit for a quick look. Quite apart from ruining your firing cycle, you can & will get badly burned!
Once the kiln has finished firing leave it cool right down then carefully open the door. If it's cold where your kiln is, cold air will rush into the kiln if you open it too quickly & cause thermal shock - your casts will shatter.

You should now have a beautiful piece of slip cast ceramics which you can paint or glaze to your own tastes.

I hope this guide has been helpful but if I've made any errors or typos please let me know.

Angela



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