Forming Techniques for the Self-Reliant Potter (GTZ, 1991, 194 p.) 7. Slip casting (introduction...) 7.1. Casting body 7.2. Slip building and fiberslip

Forming Techniques for the Self-Reliant Potter (GTZ, 1991, 194 p.)

7. Slip casting

Slip casting is a specialized way of using plaster moulds for mass production of ceramics. The word “slip” means a single clay or a mixture of clays and other materials that has been mixed with enough water to make it liquid, so that it can be poured into a plaster mould.

Slip casting is a fairly new process in the history of ceramics, as it only became practical after the invention of plaster of parts 200 years ago.

drain casting

The slip casting process shown in Fig. 7-A produces a hollow item which outside has the shape of the mould. This process is called drain casting because the moulds are emptied of excess slip.

solid casting

In this method the mould forms both the inner and outer face of the cast. The advantage is, that all casts will be identical and the cast can have walls of varied thickness. Solid casting moulds are more complicated to make and they are mainly used for high quality ware and for items that need to be of uniform size. Fig. 4.4.1-D shows an example of solid casting of handles.

FIGURE 7-A The slip casting process (A; B; C; D)

A) A three-piece plaster mould is filled with casting slip. Water is absorbed by the plaster and the clay that was mixed with that water forms a layer of clay on the inside.

B) After 20-40 minutes the mould is emptied. A layer of clay, forming a vase, remains inside the mould.

C) The mould absorbs more water from the clay and when the vase is hard enough to handle the “spare” is trimmed and the vase is released from the mould.

D) The vase is placed on a board for finishing and the mould is reassembled and used for another casting.

7.1. Casting body

7.1.1. GUIDELINES FOR DEVELOPING A CASTING BODY
7.1.2. CASTING SLIP PREPARATION
7.1.3. CASTING PROBLEMS, CAUSES AND CURES

Slip casting gives potters more problems than other forming techniques. A good casting slip is often difficult to develop, and preparation of the clay slip has to be done carefully following the same procedures each time. The following factors play a role in making a casting slip:

fluidity

The clay body needs to be fluid enough to be poured into the plaster moulds, and after the clay sets on the mould surface, the remaining clay must drain out without leaving “runs” or “tears” on the inner surface of the cast. Fluidity is the opposite of viscosity; a stiff liquid is said to have high viscosity, and a very fluid liquid has low viscosity. The viscosity of a clay slip is adjusted by addition of water or deflocculating chemicals like soda ash (sodium carbonate) and water glass (sodium silicate).

density

The clay slip should contain as little water as possible. A high amount of water produces a less dense cast and increases drying shrinkage of the clay. It will also lengthen the casting time, and the plaster moulds will have to absorb more water, which reduces the number of items that can be cast per day and the life of the moulds. Water content of the slip should be 25 % to 45 % by weight of the slip. Density is measured by weighing 1 lifer of the slip. 1 lifer of water weighs 1 kg, and we say it has a density of 1; clay slip will have a density higher than 1.

permeability

As soon as the clay slip is poured into the mould, the plaster starts to suck water from the slip, and a denser layer of clay body is built up next to the plaster. As this layer thickens, it takes a longer time for the plaster to draw water from the slip, so the casting rate is lowered. If the casting slip has a high proportion of fine plastic clay, it will lengthen the casting time considerably. Therefore, casting slips should normally contain less plastic clay than plastic bodies. As a rule, a casting slip should contain 50 % clay and 50 % nonplastic materials (by dry weight).

clay body strength

The finished cast should be strong enough to handle when it is taken out of the mould, and there should be no sticking to the mould. The green strength of the clay body needs to be enough for handling without breaking.

deflocculation

Clay particles are extremely small and shaped like plates. In a plastic clay a thin film of water surrounds each clay particle and enables the particles to slide over one another when pressure is applied during forming. As more and more water is added, the clay particles start to move more freely, and gradually the clay becomes a slip. An increase of water lowers viscosity. Equal amounts of water and clay make a slip.
So much water makes the slip impractical to cast. This problem is solved by adding chemicals, which changes the behavior of the clay particles.

Clay particles behave in two different ways when freely suspended in water. If an acid is added to the water the clay particles will attract each other, and it is said that the clay slip is “flocculated”. With addition of an alkali (e.g. soda ash), the clay particles will repel each other and this state is called “deflocculated”.

Fig. 7.1-B shows clay particles enlarged approximately 100,000 times. In an acid environment, clay particles carry electrostatic charges on their surface: positive charges at their edges and negative charges on their two faces. The clay particles behave like small magnets, and the positive edges are attracted to the negative faces. This results in packing of the clay particles, and in increased viscosity.

When the clay slip is deflocculated by addition of an alkali the particles become negatively charged all over and they will then repulse one another (Fig. 7.1-C). This repulsion makes the slip fluid with less water.

thixotropy

This means the property of clay to become more viscous when it is left undisturbed. When a slip is left all night, it often becomes thick and viscous. If it is stirred, it again will become fluid. Thixotropy is useful in a casting slip because it helps the newly-cast items to keep their shape in the mould. However, too high thixotropy makes it difficult to empty the moulds. Water glass decreases thixotropy, whereas soda ash does not affect it.

FIGURE (A; B)

A) Clay particles are attracted to one another in a flocculated slip.
B) Clay particles are repelled by each other in a deflocculated slip.

7.1.1. GUIDELINES FOR DEVELOPING A CASTING BODY

As a starting point, a casting body can be developed from a plastic body. However, the body needs to be adjusted keeping the following points in mind:

- A casting body should contain approximately 50 % clay material and 50 % nonplastic materials.

- Very fine clay particles will reduce the casting rate by blocking the flow of water from the slip into the plaster mould. Plastic clays should be substituted for less plastic ones and the clay part should be made up from more than one type of clay.

- Natural clays contain soluble salts, and these will act as flocculants, increasing the necessary amount of deflocculants. Some clays prove impossible to deflocculate. The salts may come from the water, and, as a rule, you should never use recycled water from clay preparation for casting slip mixing.

- Clays containing sulfates are also difficult to deflocculate. Sulfates can be neutralized by additions of barium carbonate, usually 0.2-1 %.

- Addition of frit as a body flux may also introduce flocculants, since the frit may be slightly soluble. As a remedy try to use another frit or use glass powder.

- Clays containing montmorillonite (like bentonite) contain high amounts of colloidal silica that may cause the slip to gel and in general are difficult to deflocculate. This is called “livering”.

- The standard deflocculant is a 3 to 1 mixture of sodium silicate and soda ash. The ratio of sodium silicate should be higher for a body of low plasticity, whereas more soda ash is used for bodies containing higher amounts of plastic clay.

Sodium silicate tends to cause the cast item to stick to the mould, produces stringy slip that drains unevenly and results in a hard cast. Too much soda ash produces casts that never get hard, clay bodies that thicken up more quickly, and causes the mould to wear out faster. Soda ash should be purchased fresh, and kept in an airtight container. Old soda ash absorbs water from the air and becomes soda bicarbonate, which will no longer work as a deflocculant.

- The amount of deflocculant should be as low as possible and not higher than 0.3 % (based on dry weight of body). High amounts cause excessive wear of moulds and make the cast stick to the mould.

testing

After developing a suitable body, the right amount of deflocculant is found by doing a simple test. First, weigh all ingredients accurately. Mix the measured body with water until the body becomes just fluid. Then, dissolve a mixture of sodium silicate and soda ash in hot water.

Example:

5 g soda ash + 15 g sodium silicate in 200 ml water equals 0.01 g deflocculant for each 1 ml water. So each 1 ml of solution added to 1 kg body means an addition of 0.01 % deflocculant.

What we want to find is the amount of deflocculant producing the most fluid slip. The fluidity is tested by filling a large funnel to the rim with slip while closing the outlet with a finger. The time (in seconds) it takes for the slip to drain out after releasing one’s finger is a relative measure of its fluidity. First the run-through time for slip without deflocculants is found and then the deflocculant solution is added to the slip 5 ml at a time. With each addition the slip is stirred well and run through the funnel. Initially, the slip will run faster with increase of deflocculant, but as the concentration of deflocculant increases, the slip will become less fluid. At that point the test is stopped and the results are plotted on a graph as shown in Fig. 7.1.1-B. A line is drawn between the dots, and from this graph the amount of deflocculant with lowest viscosity is read. Normally a point a little to the left of this is used, since it is important to use as little deflocculant as possible.

trial batch

Now prepare a 10-kg test batch of casting body following the standard procedure and using the amount of deflocculant found from our funnel test. In the above test we did not find the right amount of water for the slip. This is now done by adding only 33 % water and the deflocculants to the dry body (which means a water content of 25 % of slip). Leave it for 24 hours. After that, additional water is added during blunging until the slip has the right consistency. The slip is tested by casting some items. If it works and if you have measured all components accurately, you have the final recipe for your casting slip.

ageing

A deflocculated casting slip should be used within 2-3 weeks. Carbon dioxide from the air enters the slip and this acts as a flocculant which will make the slip less fluid. The ageing can be slowed down by keeping the slip in an airtight container or by sealing the slip surface with a thin layer of oil.

grog

Grog can be added to a casting slip. It will stay in suspension in the deflocculated slip. The finished surface will be as smooth as if no grog was used. Grog is used for casting large items, especially sanitary ware. It is also used for casting of refractory kiln furniture.

FIGURE 7.1.1-B Funnel used for testing fluidity of casting slip.

FIGURE 7.1.1-C Results of a fluidity test presented as a graph.25 ml solution (0.25 % of dry body) is the right amount of deflocculant in this example.

7.1.2. CASTING SLIP PREPARATION

1) Dry all body materials and weigh them accurately.

2) Fill the right amount of water in the blunger. Gradually add soda ash dissolved in hot water, then add half of the body. After some blunging add the remaining body and the sodium silicate.

3) Leave the slip to age for 24 hours.

4) Screen it through 60 mesh. Check fluidity with the funnel test and density by weighing 1 lifer of slip. Cast a few items to be sure the slip is working correctly.

recommended procedure for casting

1 Before casting examine the moulds for clay or dust on the inside.

2 Clean dirt from the moulds with an air hose or a soft dry brush. Never use water it will damage the mould.

3 Arrange the moulds in the order of cas-ting: heavy moulds first, then medium size, then small size.

4 Join the moulds tightly together with rubber.

5 Stir the slip just before casting and check that it does not contain air bubbles.

6 Pour the slip slowly and evenly into the mould without stopping and splashing. The slip should hit the bottom of the mould.

7 Top up the mould every 5-10 minutes.

8 Let the mould set until the casting has reached correct thickness.

9 A good mould can be used 2 to 4 times per day, depending on weather conditions. Before each casting, the mould must be dried in the sun or in the drying cupboard Otherwise moulds wear out fast.

10 After correct thickness is reached, drain out the excess slip. If the slip is drained into its main container, do this through a coarse screen that will retain pieces of plaster and dry clay.

11 Then keep the mould upsidedown until the clay is stiff enough to remove.

12 Take the cast product out when the clay starts to pull away from the plaster mould.

helpful hints

Costing: In costing greenware, the guideline is to charge 10 % of the mould cost for each casting. This can be adjusted according to how complicated the mould is.

7.1.3. CASTING PROBLEMS, CAUSES AND CURES

Problem: Slip gelling in the mould.
“To gel” or “gelling” means slip becoming too thick in the mould, so that excess slip cannot be poured out.

Cause: Underdeflocculated slip.
Cure: Adjust the slip batch by adding more sodium silicate in very small amounts (to prevent overdeflocculation).

Cause: Slip that has not been mixed sufficiently.
Cure: Mix the batch for a longer time.

Cause: Too much free iron or alkaline in the clay itself, or too much organic matter in the clay (black ball clays often cause this problem).
Cure: Reduce the amount of clay causing the problem, by experimenting with variations on the body recipe.

Problem: Uneven casting thickness. This means the walls of the casting are thick and thin.

Cause: Difference in mould absorption due to difference in the thickness of a mould thick areas absorb water faster than thin areas, causing excess buildup of slip.
Cure: Check the quality of your mould. It may be necessary to change the thickness of the mould - thick areas in the mould may need to be made thinner.

Cause: Moulds are too old and are starting to harden.
Cure: Throw out nonabsorbing moulds. Average mould life is 150-200 castings.

Problem: Greenware cracking in the mould. This means the greenware cracks before it is removed from the mould.

Cause: The most common cause is too much water in the slip.
Cure: The amount of deflocculant needs to be corrected, or there may be problems with some of the clay ingredients - in this case, it may be necessary to change raw materials.

Cause: Slip with insufficient plastic ingredients.
Cure: It may be necessary to add more plastic clay to the body.

Cause: Poorly designed moulds.
Cure: If this problem seems to occur with one or two particular moulds, then the moulds are probably the cause of cracking. They may have undercuts which catch the slip.

Cause: Greenware is allowed to remain in the mould too long.
Cure: Check your casting process. Are you leaving the greenware in the mould too long after draining it? It should be removed as soon as it is stiff enough to be handled without damage.

Problem: Greenware sticking to the mould. Cause: New moulds with greasy or soapy surfaces.
Cure: Dust the inside of the moulds with talcum powder. The problem should disappear after making a few casts.

Cause: Too much organic material or large amounts of fine particles in the clay. Black ball clay often has too much organic material.
Cure: It may be necessary to substitute some of the ball clay for china clay.

Cause: Underdeflocculated slip which does not shrink and takes longer to dry in the mould, or overdeflocculated slip with too much sodium silicate (this is the most common cause).
Cure: Adjust the amount of deflocculant.

Cause: Moulds are saturated with water.
Cure: Completely dry the mould at a temperature below 50ºC. (If mould becomes too hot, the plaster will disintegrate).

Problem: Stiff and brittle greenware. This means leather-hard or dry greenware that breaks too easily.

Cause: Overdeflocculated slip. If you experience slow casting times and greenware with a sandy surface, overdeflocculation is usually the problem.
Cure: Overdeflocculated slip usually needs to be thrown out. The amount of deflocculation, especially sodium silicate, must then be reduced.

Cause: Clay particle size may be too coarse.
Cure: Slip plasticity can be improved by ageing it at least 24 hours. If this does not solve the problem, then the recipe needs to be adjusted with more plastic clay.

Problem: Pinholes in the surface of greenware or fired ware.

Cause: Trapped air bubbles in the slip. Bubbles in slip occur if the slip is mixed at too high a speed.
Cure: Bubbles in slip can be prevented if slower mixing speed is used.

Cause: The slip has too low fluidity. Cure: Increase deflocculants and/or water.
Cause: There is a high carbonate content in the water used to make the slip.

Cure: It may be necessary to use water from a different source.
Cause: There is a high content of organic material in the clay which releases gas when it decomposes. Especially black clay contains a high amount of organic materials. Cure: Screen the slip through a 60-mesh sieve. This will release the trapped gas.

Cause: The soda ash in slip decomposes because of too long storage time.
Cure: Throw out the slip.

Cause: Fast pouring into the mould may introduce air bubbles and the return of excess slip after casting may do the same.
Cure: Pour the slip slowly without stopping until the mould is full and stir the slip before casting to release air bubbles.

Cause: Poor quality moulds with pinholes on the surface.
Cure: Improve mould-making process, by mixing plaster more carefully to prevent any air bubbles.

7.2. Slip building and fiberslip

There are a few specialized ways of working with slip that are not found in most ceramics books: one is using deflocculated slip for joining plastic clay, and the other is making large slabs by mixing fiber into the slip, called “fiberslip”.

As already mentioned in the section on plastic clay, there is an advantage in using deflocculated slip for joining leather-hard clay, because the shrinkage is the same as plastic clay - this is especially true when joining large pieces, which tend to crack at the seams in drying. The process is:

- Mix deflocculated slip, using the same body recipe as your plastic clay. This type of slip is most easily deflocculated with “Calgon” (sodium hexametaphosphate) in amounts up to 1%. Calgon is mixed first with water, and then the clay body is gradually added while blunging.

- When joining, it is not necessary to scratch the surfaces to be joined (this may not be true with all clay bodies, and needs to be tested). Apply slip to both surfaces with a brush, and press them together immediately. Excess slip can be removed from the seams after it has set a bit.

- For extra reinforcement of the joints, they can actually be “taped” together. The tape is made by cutting strips of very open weave fabric, such as cotton mosquito netting or cotton gauze; or if available, a very open weave fiberglass cloth. These tapes are dipped in slip, and applied to the joined pieces. The tape provides very high green strength and resistance to drying cracks. In the firing, it burns up, leaving only the slip. The disadvantage of this method is that the tape produces a texture at the joints - however, if used on the inside of items such as large planter boxes, this is not a problem. It also is possible to incorporate it as decoration on the outside.

building forms with slip

It is also possible to build with slip, using moulds. This process is suited for making slabs, or for producing forms that are too large for practical casting in the normal way:

- Deflocculated slip is prepared as usual, but as little water as possible is added. The slip should be just thin enough for the mixer still to work successfully - it will not pour as easily as regular casting slip. It can be used like this, or, for extra strong and more easily workable slip, it can be mixed with 0.5 % chopped nylon fibers - these are sometimes available in bulk, and should be cut to about 2 cm. It is important that they be single, unwoven fibers. It also is possible to make these fibers yourself, by chopping nylon rope. Nylon fibers work the best, because they are very fine and easy to separate. One half percent seems like a very small amount, but it is surprising how it increases the viscosity of the slip, and it gives incredible green strength.

Alternatively, finely-chopped jute fiber can be used. In India, this is often used in unfired clay sculpture.

- Building with this “fiberslip” is best done inside plaster moulds. The slip is simply spread on by hand or with a trowel, and built up to the necessary thickness (which will be about the same as plastic clay). The advantage is that slabs do not need to be made, and there are no problems with joining. Drying shrinkage is less than plastic clay, and cracking problems cannot occur because of the fiber. The nylon fiber is so fine that even though it burns up in firing, the finished appearance is no different from regular clay.

- Because the green strength is so great, it is a useful technique for large pots that are difficult to transport when green.

fiberslip for sculpture

Another way of using fiberslip is for slab making or for sculpture. Again, this is a process that ordinarily would not be used, but for large-scale work, where there are shrinkage problems, it is worth trying:

- The same slip as above is used, preferably with the addition of chopped nylon fiber.

- The process of building up slabs is the same as making fiberglass, where glass cloth is used to reinforce plastic resin. With fiberslip, various kinds of cloth can be used: the main requirement is that they be very open weave so the slip can penetrate through them. Fiberglass cloth is good if it is about the same weave as mosquito netting; nylon net or cotton mosquito netting can be used; or “cheesecloth” (open-weave cotton gauze) also works well.

- To make a slab, first a paper (newspaper) separator is laid on a flat surface. Next, a piece of cloth the desired size is laid on this. Then, a layer of slip about 2-4 mm thick is spread on the cloth. Another piece of cloth is laid on the slip, and the process is continued until the slab is the desired thickness. When the slab is leather-hard, it can be cut (with a sharp knife) and assembled with slip and tape as above.

- For making large sculpture, the process can be quite useful. An armature (support) can be made from wood, wire, etc. and then covered with wire netting. This is then covered with cloth, and fiberslip is built up on the surface as for making a slab. Usually, the outer layers will be fiberslip only (with no reinforcing cloth) so that the surface can be caved and modeled. The pieces can be fired together - the armature will burn and melt in the process, but as it is inside the sculpture, this does not present a problem. Clay with low shrinkage is best for the process, such as plastic clay with a large grog percentage, or low-firing white clay such as 50 % ball clay/50 % talc.