Monday, 7 May 2012

Making a plaster cast of a fish

When my friend, Linda Scott-Robinson's husband, Tony asked me to make him some ceramic salmon for his garden, I have to admit that I was less than enthusiastic. I don't know why, but I've just never been a big fan of making pot 'creatures'. However, I said 'okay' and set about experimenting with the shapes and colours of river salmon. Tony drew the shape he wanted and I made a flat test piece directly from his drawing. I covered it a right old mix of glazes and suddenly began to feel a lot more curious about what could be achieved with this here fishy business...

I knew, of course, that the thing needed to be more dimensional and robust to survive the rigours of life in a North Northumberland country garden. Tony (being an award-winning fisherman!) duly supplied me with a real (dead of course!) salmon from which I could make a plaster cast... Oh heck.... I definitely didn't fancy that idea!!

Linda dropped off the fish at my office and I stored it in our staff kitchen fridge icebox until home time, much to the amusement of my colleagues who seemed to think it was an excuse for a 'stream' of pretty bad fish-related puns... On getting the fish home, I quickly put it into the freezer and tried to forget about it.

After a week of it being there, I realised that I'd have to face up to the task of casting it. I got it out of the freezer on Friday night and placed it in a long tub of cold water to defrost... in the garage... On Saturday morning, I had a tentative look at it. It was slimey and horrible and still a bit frozen but it didn't smell much, so I proceeded to rinse it with fresh water and gently clean the slime off, taking care not to damage the skin. After several flushings with cold water and a good pat dry with a bath towel, it looked like it might be dry enough to cast.

Here's a list of all the things I used when casting the fish:

· large quantities of any old poor quality, softish, reclaimed clay 
· 20 pints of water 
· 10 kilos of casting plaster 
· large wooden spoon, large plastic whisk
· large bucket, glass jub
· various pottery tools - loops, ribs, kidneys, scoops, wires, pins...
· long strip of lino
· strong tape and garden wire 
· plastic-backed cloth 
· old towel 
· dust mask

Here's the steps I took:

1.      Lay a large slab of clay on to the cloth. The clay should be as long and wide as the fish with an additional 3 inches all the way around. Make it as even in height as possible across the full length.

2.      Trace the fish shape on to the clay and begin to scoop out enough clay so as to be deep enough to insert half of the fish along its length into the clay slab:

3.      Keep testing the size and shape of the hole and scooping out more clay until the fish fits perfectly, with exactly half of its body above the clay and half inserted. Finally, lay the fish into the clay slab and carefully spread the fins and tail, gently pressing in so that they too are half immersed in the clay. Ensure that the area around the fish-shaped hole is smooth and flat (the flatter and more even it is, the tighter the finished mould will be):

4.      Watch out for the fish sagging over the edge of hole - and compensate for this by cutting the hole bigger if necessary. Sagging on to the top of the clay will create an 'undercut' and make it difficult - or even impossible - to remove clay work from the finished cast.
I really don't like this image, but it does illustrate the saggy nature of a dead fish !

5.      Create locating holes by pressing the handle of a turning tool into several areas of the clay surface. Create a solid 'funnel-shaped' piece of clay and cut this in half along its length, fit on to clay surface flat side down, as shown below. This will create a funnel into which casting slip can be poured (mine is probably a bit narrow). Note in the background all the various tools used in scooping and tidying the clay slab:

6.      Now surround the whole thing with a strip of lino that is tall enough to allow for at least a 3 inch thickness of plaster to be poured on top of the fish.

7.      Secure the lino with tape and/or wire or string. Push plenty of clay up against the outside of it to form a tight seal against the clay inside. My lino was a bit floppy because it was brand new, so I added some wooden boards for support. Using an older, stiffer lino or some metal flashing would have been better.

8.      Time to add plaster... I use the 'island method' for making plaster. I decided that I needed about 10 pints of water to cover the fish and build up the 3 inch thickness. I put all of the water into a bucket, donned my dust mask and then scooped in plaster bit by bit until there was enough plaster in the water to form a little island of dry plaster above the surface of the water. I then gently stirred the plaster with the plastic whisk and wooden spoon until it was smooth and creamy, but not bubbly. Once the plaster began to thicken a little, I poured a thin layer over the whole fish, and ensured a good covering. (Some horrible mucus seeped out of the gills during this process and floated on top of the plaster - I had to remove this carefully by simply laying a sheet of kitchen roll on to the surface of the plaster to soak it up... yuk). I gradually added all of the plaster until the bucket was empty. It's important to get all of the air out of a plaster mould otherwise you end up with holes that can spoil the finish and even cause the mould to crack or break. Gently bang the workbench and sides of the lino to encourage bubbles to burst. Now leave the plaster to set. I don't know how long it should be left for! I'm an impatient sort of person... I left it about half an hour, then removed the lino and flipped the mould upside down and gently removed the clay to reveal a fish embedded in plaster! (It wasn't stuck)

9.      Now to make the matching cast of the other side. Gently clean any clay or other debris from the fish. Add the other half of the pouring funnel, then coat the entire cast (not the fish!) in a thin layer of clay slip using a brush and ensuring that no bare patches of plaster are showing through the slip. Once again, enclose the whole cast in lino and seal tightly leaving at least 3 inches of thickness for plaster above the fish. Make up another bucket of plaster and add it to the mould pretty much in the same way as before. Leave to set. Remove the lino and you will end up with something looking like a fish sarcophagus:

10.  The next bit is tricky... separate the two halves of the mould. Depending on how careful you've been with the slip between the moulds, you may need to work a groove between the two halves in order to separate them. (I ended up shoving a kitchen knife between to the two pieces to break them apart and, in the process, chopped off one of my 'locating nuts'!)

If all goes well, eventually the two halves should come apart revealing a fish-shaped mould ready for use...

I'd like to say that no fish were hurt in the making of this mould, but that wouldn't really be true. Admittedly the fish was already dead by the time I got it to make the mould.
I did feel a bit sad though - it was a lovely looking fish.

Definitely a one-off experience for me!


Special thanks to Paul Allen from Newcastle College who taught me everything I know about how to make a slipcast mould.

Sunday, 6 May 2012

Rolling clay slabs

There are a number of seemingly straight-forward tasks that can cause all kinds of problems for a student potter.  For example, the making of large, good, flat slabs without the luxury of a mechanical slab roller can be a real pain.  Apart from the sheer effort of bashing several kilos of clay into submission, it is always a challenge to ensure even thickness, and bubble-free, perfectly flat surfaces that wont distort under firing. However, when making large slabbed work, the preparation of the clay is critical to success so it is worth making the effort to get things right.

One of the first problems with working with anything bigger than fairly small-sized slabs is that the clay has a tendency to stretch under its own weight when you flip it over between rollings...

To get around that, I've learned to always, always roll out clay on a sheet of cloth.  This also serves to keep the clay clean and grit free (especially important if working with porcelain) and prevents it from sticking to the workbench.  I've found that the best cloth for this purpose is flock-backed cheap plastic tablecloth (the kind that can be bought from Poundland in quantity! Thank you to Jess for that top tip.);  this can be cut to size and washed and reused many times.  

Before beginning to roll out the slab, I stop and think about the desired final shape of the slab - is it rectangular, square, round, other...  ?  Whatever shape I want, I try to start with a block of clay that looks like it might roll into that shape! From the bag, I wire off a large lump of clay, trying to avoid tearing or folding the clay. I then bash this into shape using a combination of repeatedly dropping on it to the workbench, thumping with my fist and patting into shape, again avoiding tearing or folding. Depending on the size of slab I want, I may cut this larger lump into multiple thick slabs. If the clay is uneven in thickness to start with it, it will stretch and bulge in all the wrong places when rolled.  The ideal tool for cutting thick starter slabs is a metal harp as this gives a lovely, controlled, even cut:

Once I've got my clay ready, I lay it on a piece of cloth that is twice is as big as the slab I'm going to roll and begin rolling. I use a long straight-sided rolling pin. A pair of wooden slats of even height serve as guides. (It pays to have a good supply of pairs of wooden slats of varying heights.)

It's necessary to frequently turn clay between rollings in order to ensure optimum and even stretching without dragging or sticking;  this is where the tablecloth comes in very useful. To flip the clay, I fold the tablecloth in half over the clay and then flip the whole thing over, peel back the cloth and carry on rolling the clay on the other side:

The use of the cloth in this way is extremely effective with really large slabs as it maintains the shape and prevents unwanted stretching between flips.  If the clay is particularly sticky it will stick to the rolling pin, so it might be necessary to roll it sandwiched in between the cloth - i.e. just roll on the cloth rather than the clay.  However, it's better if the clay isn't so damp that it sticks to the rolling pin; keeping the rolling pin wiped cleaned between rolls will make this less likely to happen too.

So that's my technique for rolling slabs; other potters may have better methods but that'll work fine for me until I have room to accommodate a proper mechanical slab roller!

For my next trick - making a plaster cast of a 20 inch salmon...  watch this space....