Teacher Information | Next | Index |
To complement the residency the teachers organised visits to Gladstone Pottery Museum and the Royal Doulton factory.
The sculpture she made was a scene from a pottery factory in the last century. It shows a man handing a plate to a paintress for decoration. This job was often done by children at that time. Now lets go back to the beginning and see how it was made.
Section 1 : SLIP CASTING | Next | Previous | Index |
Mould making can be quite complicated depending on the shape of the figure [or article] to be moulded. If it's wrong then it locks together as a jigsaw and you can't get the cast out without breaking it. Quite often a figure is made up of more than one part, from different moulds, and the parts are stuck together using liquid slip. Jugs and handles are often made separately and then joined together later. The moulds don't last very long because they get chipped easily round the edges. Usually you get about 20 or 30 casts depending on the detail.
The slip needs to be well stirred before use and you need to have enough ready to fill the mould right up to the top. Elastic bands are used to hold the mould together. The ones you see are made out of car inner tubes. As the plaster soaks up the water, the level drops and you can see the clay wall building round the top of the pour hole. This needs to be 3 or 4 millimetres for a jug and 5 millimetres for a plate. If the mould is already damp it takes longer because the plaster takes up less water. The next step is to pour out the slip. Sometimes we need to poke in a stick if the hole is so small that it blocks up. Then we need to wait at least 15 minutes for the inside of the mould to go solid. We need to make cuts around the top to let the extra clay be taken off.
This jug has a dent in it because some of the slip has not drained out, and so it has not had chance to dry properly. We need to smooth off the top of the jug with a damp sponge. If the mould has not been filled up all at once, then you get a little ring going all the way round, which shows up on the finished pot. When the mould has been used a few times the edges get damaged and you end up with seams that need scraping off. Cutting off the seams and smoothing the edges is called "fettling". The clay starts off shiny and goes dull as it dries out. As it dries, it shrinks and when it is fired it shrinks again, so it will end up at least 10% smaller. This means that the mould has to be bigger than the size you want to end up with.
The edges and inside of the mould must not be touched with metal or plastic, as these materials are hard and can cause damage. The mould needs to be cleaned with a damp sponge, and must dry out properly so it can absorb absorb more water from the next casting. The mould should be left as long as possible before opening, or the clay will be too soft and will bend. You should not pour slip over the string, because the bits will fall off and stick to the wet clay. The Romeo casting is one of Angela's moulds and it's quite a complicated one in several pieces. You shouldn't really need to use a hair dryer but it was used here because it was almost lunch time. It would have been better to leave it a few hours or overnight. For the tipping and draining, a shelf from an old fridge can be put on top of a large plastic bowl.
When the plate has dried out so that it is stiff enough to lift safely, it can be taken to the kiln for firing. In a kiln the temperature is raised very slowly to 200 degrees centigrade, then slowly (50 to 75 degrees an hour) up to 950 to 1000 degrees for the first firing. After the first firing the plate looks dull because it hasn't been glazed yet. It is called 'Biscuit ware' probably because it breaks easily. Biscuit ware is very porous which helps the pot to absorb glaze more easily . The higher the temperature the more glassy and hard the clay becomes. The final firing for this type of clay after the glaze is applied is at 1040 to 1060 degrees. Decoration can be applied to the Biscuit ware either before glazing, called 'underglaze decoration', or after glazing, called 'on-glaze decoration', or both.
Slip casting is a lot faster than a potter's wheel when a lot of pots need to be made as several casts can be taken at once if several moulds are available. Also it has the advantage that all the castings are exactly the same and that's important when you're making a set of dinner plates, for example. This method is suitable for mass production and has changed pottery making from a craftsman's trade to an important industry. The pottery industry started around Stoke on Trent because it had lots of clay nearby and there was coal as well for the furnaces to fire the pots. The canals were used to take the finished pots to markets or to ports for export.
Section 2 : PLASTER CASTING | Next | Previous | Index |
Plaster must be mixed with the right amount of water. If there's not enough then it will be too stiff to pour into the mould; if there's too much it won't set hard enough. We usually need 1 litre of water for each kilogram of plaster but there are different kinds of plasters so more or less may be needed. We must always add the plaster into the water, not the other way round. If we're not careful the plaster will stick to the container and we won't be able to get it out when it sets, so we coat it with vaseline first. Plaster can't stick to vaseline. Plastic or polythene containers are best as the plaster does not stick to them easily.
When plaster goes into water there is a chemical reaction and little bubbles are made. So we need to stir it until the bubbles have disappeared. If we leave the bubbles there we get little holes in the cast. The lumps need to be pressed out on the side of the container with a spatula. Another way is to add the plaster to the water until it lies just below the surface after all the water has soaked into it, and then start to mix it. A little salt can be used to speed up the setting process.
When making footprints and handprints it's best to do the footprint in clay and hand print in plaster in a plastic container. Clay needs more pressure and that's easier for the footprint. The police often take casts of footprints at the scene of a crime. For a footprint we need a slab of clay that is at least 2 cm deep so that the footprint gives a good depth of plaster. Extra plaster can be poured onto the top of the mould to build up the thickness to at least 1 cm, otherwise it will be too thin and probably get broken. If the first footprint isn't very good you can turn the clay over and use the other side. Check the shoes for dirt before treading in the clay. If the soles of the shoes have soil on them it gets into the clay, and if this happens don't try to remove it. The cast will be spoiled if you put the foot down twice or move it when it's in the clay.
There are two ways to make a hand cast. We can press it into clay like the footprint but it needs to pressed very hard. If we do it this way then we need to make a little clay wall where the wrist was, to stop the plaster running off. A better way is to put the hand into some plaster in a bowl. With this method we can make a mould in two halves and do a complete hand. We pour two cms of plaster into a plastic container that has been greased, and is big enough to give a 2 to 3 cm gap around the hand, and then place the hand on the surface. The hand needs to be rubbed with vaseline to stop the plaster sticking. The hand should be gently pressed halfway into the plaster just as it is beginning to set. The plaster must not come over the top of the hand, or it will get stuck and the mould will have to be broken to get it out.
The wrist needs to be as close to the corner as possible so that the plaster does not go too far up the wrist and make it difficult to get the hand out of the mould. To stop the plaster going all round the wrist a piece of clay needs to be stuck into the corner to hold it back. The two halves of the mould need to fit together accurately to avoid getting a mis-shaped hand or big seams on the finished casting. Angela makes hollows in the bottom half so that matching bumps will be created when the top half is made. These are called natches. The natches are made just before the plaster sets.
When the first half of the mould has set, any plaster round the edge of the container should be cleaned off first, to stop bits dropping into the second half. It is then smeared with vaseline or a special liquid soap called mouldmaker's size, so that the two halves of the mould can be separated. The hand is put back and plaster is poured over the top to make the second mould. When the other half of the plaster mould has been made, it can be assembled and used for slip casting another hand. If you are the model for a cast you need to be comfortable to avoid movement when the plaster is setting.
Before using the mould for slip-casting the vaseline needs to be washed off to let the plaster absorb the water. It needs to be sponged gently with some washing up liquid. The mould should be left to dry for several days in a warm place to soak up the water out of the slip. There are many different kinds of plaster. There is hard plaster and soft plaster and coloured as well. Rough plaster is used for walls; it's got sand in it. When we're clearing up we must not put any plaster down the sink because it sets and blocks up the pipes. It's best to put it in a plastic bin. Hands covered with plaster should be washed in a plastic bowl and not in the sink so the plaster can be thrown away separately in a dustbin.
Section 3 : LIFE CASTING | Next | Previous | Index |
The first step is to put vaseline on the face and cotton wool in the ears. The eyelashes and eyelids must be covered. A rubber swimming cap is put on the head to protect the hair. Plaster is not good for casting a whole head because it is too hard, and could be painful and difficult to remove, especially round the ears. Also it can irritate the delicate skin of the face, so a different material called 'alginate' is used. Alginate is made from seaweed and it's like liquid rubber. It's used by dentists for casting false teeth. It doesn't harm the skin or cause rashes so its good for life casting.
Dental alginate smells like peppermint. Alginate is different from plaster because the water is added to the powder. (For plaster it's the other way round. ) If warm water is mixed with the alginate then it sets faster, within 5 minutes. When set it stays flexible and the mould can be taken off easily. Everything must be ready before we start, as the mould has to be made as quickly as possible. There's no turning back. Graham won't be able to talk when the alginate is over his face so he needs to give hand signals to tell Angela he's OK. The plaster bandages stiffen the mould to stop it losing its shape when it's taken off. They're used in hospitals for setting broken arms and legs.
The mould is made in two halves so that it can be quickly removed, and Angela works very fast so that Graham does not have be trapped in the plaster case for for than a few minutes, because it gets uncomfortable. After taking off the mould it is strengthened with more plaster bandages and hessian. The holes in the alginate where the nose was have to be blocked with clay to stop the plaster running through and sticking to the plaster support. When the moulds are ready the two halves are put together again for casting the head. The head needs to be cast hollow so it can fit over the metal frame.
A fine high density plaster is used as it is harder than potters plaster, and finer so it picks up the details better. It needs less water per weight of plaster when mixing. After pouring in some plaster,we make sure the plaster has covered the whole of the mould by swirling it round. the mould is then knocked to release the air bubbles. The process is repeated until a layer is built up inside the mould at least 2 cms thick. The other method for making a hollow casting for a head is to cast the two pieces separately and then stick them together. A layer of plaster is added first and then strengthened by mixing plaster with chopped fibreglass or hessian. Extra plaster is wiped off the edges of the mould with a cloth.
After setting and hardening, the two halves are fastened together and plaster added to the inside to fill in the join, to make the complete head. Denise has made a cast of the man's head. She needs to be very careful taking the head out of the mould because the ears are easily damaged. If there are little lumps on the face, this happens because there are holes in the rubber where the plaster can get through. All the lumps need to be carved away. The mould can be used to make face masks. Plaster can get quite hot while it's setting. There is a seam going all the way round where the two halves of the mould were joined together. That must be scraped off. The swimming cap has left an impression as well so that needs to be scraped off. This process is called fettling. It is easier to do the fettling just after it comes out of the mould because the plaster is still fairly soft. After a few days the plaster gets harder and more difficult to work on.
To finish, extra plaster needs to be put on the top of the head to make up the thickness of the hair. Because the mould was made with the eyes closed the top lid needs to be filed off to make it look as if the eye is open. The next step is to make the hands in the same way, by moulding and casting. For this a mould must be made. Alginate is good for this job because it's flexible enough to let the arm come out without having to make a mould in two parts. The hand needs to be coated with vaseline again, but not too much or the detail will be lost, and the alginate poured round it and kept in place by a suitable container. In this case a tube was used.
When the alginate has set, after a few minutes, the hand is carefully pulled out leaving a mould of the hand and wrist ready for the plaster to be poured in. When the moulds are ready plaster casts of Graham's hands are taken Plaster is mixed with chopped fibreglass for strength. This is high density plaster and it sets very hard. Before the plaster has set, some wires are poked in. These are 4 mm square aluminium and they fix the arms onto the metal skeleton. Sarah's hands for the girl statue, are done in the same way as the man's.
When you're mixing alginate you add 3 parts water to 1 part powder. It starts to go runny when you mix it. All the lumps need to be mixed out. You can tell when the alginate is ready because it peels off without sticking. The alginate needs to be cut away very carefully because the fingers are easily broken, before the plaster has hardened completely. The next job is to make castings of Sarah's boots. Alginate is used again, as it wont harm the leather. This mould could be made by putting alginate all the way round and cutting it afterwards but Angela wants to do the mould in two halves.
A clay wall is made before putting on the plaster, so that the two halves of the plaster mould can be separated. The walls are positioned at the ankle bone. To make the two halves of the mould fit together we need to use natches. You can buy natches made out of blue plastic The alginate is quite floppy and bends easily so it needs to be supported by a plaster casing. When the outer casing is dry the mould is carefully separated and removed from the boot without any damage. A cast is made of each half of the boot separately so that it can be fitted round the metal armature. When weighing out the plaster, don't forget to subtract the weight of the container.
If warm water is used for mixing the plaster it sets quicker. After setting, the plaster cast is removed and takes several days to dry out completely. The thicker it is, the longer it takes.
Section 4 : PHOTOGRAPHY | Next | Previous | Index |
The photographs will help Angela to work on the head when the girl is not there. The pictures must be taken from the same height as the head and from different angles, because the head needs to look right from the sides as well as the front. This tape measure has a metal clip on the end so the 10 cm mark is used and the readings reduced by 10 cm.
Section 5 : GIRL'S HEAD MODELLING | Next | Previous | Index |
A good way to do this is with a large pair of callipers. Several measurements are taken, for example, the width (from ear to ear), the depth of the head, the width across the temples, distances between eyes and nose. Sometimes the police employ sculptors when they find a skeleton and they want to know what the person looked like. A good reference is the hole in the ear to the tip of the chin.
When they are making a statue at Madame Tussauds they take a lot of photos, some under the chin and on the top of the head. Sometimes the sculptor models from photographs because the subject is often very famous and doesn't have much spare time, so they can only be there for the finishing touches. The hands are cast, not modelled. They have a drawer full of artificial eyes in the sculpture department and they match up the colour and put them in the sockets.
If the head was moulded in clay and then fired it would shrink by about 10% during the drying process and be the wrong size. So Angela is going to mould the clay and make a plaster cast of before it dries out and shrinks. Then the cast will be the right size. Often a sculptor will make the neck a bit longer because it looks more elegant. The eyes are quite dark so the sculptor sometimes makes a hole to get the same effect. With this head the eyes will be solid because they are going to be painted.
If the head was going to be fired it would be cut in half and separated from the wooden support. Then it would be hollowed out until the outer layer was about 2 cms thick. The two halves would be joined together again with liquidised clay and allowed to dry out for several weeks before firing. The divider this time will be a clay wall instead of the plastic divider used for the live cast. The walls need to be about 1 cm thick. The wall is a different coloured clay so that the joint can be seen better when it is separated afterwards. An alginate mould is quite soft and has to be supported in a plaster casing. Clay walls have to be made to stop the alginate running away.
The walls should really be left to harden for a while before pouring the alginate. Now you can see what happens when the clay walls are not hard enough. Angela cuts a dovetail pattern into the alginate. This helps to lock the alginate into the outer plaster support As the alginate dries out it shrinks, so it must be kept damp if the mould is not used straight away. Once the clay has got bits of plaster mixed with it then it can't be used for any pots that are going to be fired because they would crack in the kiln. It doesn't matter if there are any bubbles now because the surface layer has already been put on. The mould gets quite warm as the plaster sets.
Section 6 : ASSEMBLY | Next | Previous | Index |
The body is made by fixing wire mesh onto the armature and then padding it out with newspaper. Then it can be filled with plaster mixed with a substance called vermiculite. Vermiculite is a natural mineral that is very light and puffy. It is sold at garden centres for mixing with compost to help with drainage. When it's mixed with plaster it's like dough and is much firmer. The boot has to be cut in two to fit it round the armature and then glued back together using superglue. You need to be careful with superglue because it can stick your fingers together.
The boot is filled up with plaster and then laced up before the plaster sets. Once they are attached, the arms and legs are built up on the armature with more plaster.
Section 7 : OTHER ACTIVITIES | Next | Previous | Index |
Note: This section shows how the educational experience of the Residency can be reinforced by complementary activities. The children`s knowledge is tested by placing them in an interview situation which concentrates their attention and interest. At one point a pupil makes up a word (localist) so the teacher can hand out a dictionary to one of the pupils to look it up. Then the teacher can ask the class what word should have been used instead.
Section 8 : FINISHING TOUCHES | Previous | Index |
Assistant: Denise Dutton
Narration: Alex Robinson
Replica Materials Ltd
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Alec Tiranti Ltd, 70 High St, Theale, Reading RG7 5AR Tel. +44 (0)1734 302775
Potclays Ltd, Brickkiln Lane, Etruria, Stoke on Trent, Tel. +44 (0)1782 219816