Mould making for ceramics

The secret of successful slip casting reproduction lies in the proper making of plaster moulds. As pointed out in the part on slip casting, moulds are made of plaster of Paris. It is well, therefore, that we discuss plaster of Paris, what it is, and how it is handled to obtain the best results.

Plaster of Paris

Plaster of Paris is one form of gypsum-calcium sulphate in chemical combination with water-a common mineral existing in several forms, which was in ancient times and still is employed to make plaster of various kinds.

Pure gypsum is white and relatively soft. When gypsum is exposed to high heat (calcination) so that about 75% of the "chemical water" is driven off, the resultant material is called plaster of Paris. This material is white, is easily pulverized, and combines quite readily with water at normal temperatures to form a hard white porous plaster.

The name plaster of Paris derives from the quarries at Montmarte, near Paris, France, where natural gypsum was extensively mined. However, gypsum exists in all parts of the world, from which plaster of Paris can be made.

Plaster of Paris is readily obtainable at any art or ceramic supply house and is quite inexpensive. Be sure to specify that you wish plaster of Paris for ceramic mould making.

When plaster of Paris powder is first mixed with the proper proportion of water, it becomes a soft plastic mass, which can be moulded, or cast in various shapes. After a short time it hardens into shape and is quite rigid and strong. In fact, the addition of very large quantities of water to the plaster object after it has hardened or dipping the plaster object into water will not cause it to dissolve, soften appreciably, or change its shape. After the plaster has hardened it can be trimmed with a knife, scraped, or sandpapered to make minor changes in its shape. However, when moist, the plaster can be trimmed with a knife perhaps a little more easily. When the hardened plaster is quite dry, it has the quality of absorbing water readily, without losing its shape. This quality of plaster of Paris to absorb water is most valuable, as explained in the part on slip casting.

The procedure for properly preparing plaster of Paris will be covered in lesson VIII.

General Information

The most important principle, in making a mould, is to design the plaster pieces which form the cavity so that the object which is cast in it can be removed without damage either to the mould or the casting.

Fig 26

An example of what is meant by this principle is illustrated in the sectional drawing (Fig. 26). In sketch A, a bowl is cast in a one-piece mould. Obviously when the clay dries and shrinks, the casting can be removed. However, if the simple vase were cast in a one-piece mould, as in sketch B, it would be impossible to remove it.

It is therefore necessary in determining the pieces (and their location) that are needed for a mould, to insure that there is no "undercutting" in the design of the model. If there is undercutting, when the plaster is formed around the model the pieces cannot be lifted away.

In making mould pieces, the plaster is formed around an original model. This model must be nonporous. If any porous section is exposed to the plaster, the plaster will run into the pores, and after hardening will make it most difficult, if not impossible, to remove the model. If the original model is wood, or bisque, it must be shellacked or varnished to close its pores. If the model is plaster of Paris, it must be shellacked with three coats of shellac. If it is a glazed (all over) ceramic piece or is of glass, no further action need be taken with the model.

Models may be of plasteline or modelling clay (clay mixed with Vaseline), in which case they need not be shellacked, but in all likelihood they will be damaged or destroyed after the mould is made. They can be used only with considerable skill. Many ceramists make original models of plasteline, but are indifferent to what becomes of the models-just as long as the mould is correct-from which duplicate models can be cast.

If plastic clay is used as the model or part of the model directly, it should be quite moist. Such plastic clay models must be solid clay through and through.

Plaster of Paris, when hardening around a model, does not shrink, as does casting slip clay when it hardens. The cavity which is formed in the plaster is identical in size with the original model.

In making a mould, arrangement must always be made for an opening or gate into which slip can be poured for making castings. The making of such a gate is described in the lesson on two-piece mould making, Lesson IX, Part I.

The plaster mould must have a thickness of not less than 1 inch at any place. This is necessary in order to provide sufficient plaster body to readily absorb moisture from the clay slip, during casting.

Plaster of Paris will adhere to some extent to the shellac coating or other types of surface when formed upon these. It is therefore necessary to coat the model with a "separator." In the following lesson we describe the procedure for applying a soap film to the model. This soap film is the separator. The use of soap as a separator also facilitates the removal of the model from the mould, after the plaster hardens. Brown laundry soap is an ideal separator; do not use other types of soap.

Plaster of Paris powder absorbs moisture readily from the air. Always store this powder in an airtight container.

Lesson VIII

Part 1. Preparing plaster of Paris for mould making

Materials:

  • A 10-pound sack of plaster of Paris
  • A large wash basin
  • A large metal soup spoon

Note: The proper mixing of plaster of Paris is of the utmost importance. It is the essence of successful mould making. If the plaster is improperly made, the plaster may harden too rapidly, may flake, or may form air bubbles on its surface. Follow the procedure below exactly and carefully.

Procedure: Fill the wash basin about half full of water and set on a level surface. With the hand, lift out powdered plaster of Paris from the sack or container. With a gentle sprinkling motion, sifting the powder between the fingers, permit the powder to fall evenly all over the surface of the water. With the thumb rubbing across the fingers break up any lumps of powder, and let the particles fall between the fingers. Add powder in this fashion until the water is completely filled. Add more powder until it just appears in a dry state above the surface of the water, indicating that the water is fully saturated with powder. Do not disturb the water, nor stir the powder in the water during this sifting process.

Permit the powder to remain undisturbed in the water for 2 minutes. Now with the spoon, stir the mixture very thoroughly but slowly from the bottom up and around, to release any air bubbles which may have formed in the water. Stir evenly but not too quickly, until the mixture begins to thicken. The plaster is ready to be used. It now may be poured or spooned around the model. See the next two parts of this lesson on applying plaster of Paris.

After all the required plaster has been used, the plaster remaining in the wash basin should be washed out into a large container. This container should be emptied on the ground outside; never in the drain. It will quickly stop up the drain pipes. If the plaster hardens in the wash basin, it may be broken away and the scrap pieces thrown away. They cannot be used again.

Part II. Making a plaster bat

Materials:

Same as part I, this lesson, plus:

  • 1 paint scraper 2 1/2 inches wide
  • 4 sticks of wood 15 inches x 2 inches x 1 inch
  • 1 piece of wood 18 inches x 18 inches x 1 inch
  • 1 stiff hair brush
  • 1 soft paint brush, 1 inch wide
  • 1 bar of brown laundry soap (do not use toilet soap of any kind) Shellac and paint brush
  • Nails or wood screws

Fig 27

Procedure: Secure the sticks of wood on the 18 inch x 18 inch square of wood with screws so as to form an open topped box 12 inches square on the inside and 2 inches high (Fig. 27). Paint the interior of the box and the wood square with two coats of shellac. Set on a level table.

The separator is now applied. Wet the 1-inch paint brush and work up a lather with the brown soap. With the brush, heavily soap the bottom and sides of the box. Squeeze out the lather from the brush with the fingers. Pass the brush over the soaped areas of the box to remove all suds, leaving only a soap film. Make sure no soap bubbles remain on the box.

Mix half of a wash basin of plaster of Paris, as indicated in part I of this lesson.

When the plaster begins to thicken, spoon it into the inside lower corners of the box and along the inside edges. Now pour plaster from the basin into the box until it flows to a level of 1 inch in height or more. With the paint scraper, approximately level the soft plaster. Do not run the scraper more than once across at any place. The plaster will level itself.

As the plaster begins to "set" it suddenly commences to get warm. (It does not get above comfortable heat to the hand.) Let the plaster set for 15 minutes. The wood screws may now be removed and the box disassembled.

The plaster bat remaining is sandpapered to a smooth surface and the edges rounded. It is ready for use, as indicated in previous lessons.
Hints: If the scraper is passed across the plaster (while soft) several times at any place, it tends to pack the plaster at that place and make the plaster less porous and absorbent. Do not putter with the plaster, or work it over and over again with the scraper. Single smoothing strokes once across are sufficient.

The plaster bat is useful for clay working because of its moisture absorbing quality. Pieces of moist clay can be placed upon it, and the bat will draw out the moisture more quickly than if the clay pieces were exposed to the air only. In drying flat pieces of clay, such as those cut out in the slab method, the clay surface closest to the bat will dry more quickly than the other side. Therefore, turn the clay piece over from time to time for even drying on both sides.

A circular plaster bat may be made by the above procedure, using a wooden or metal loop instead of a rectangular form.

Part III. Making a flood mould

Materials:

Same as part II, this lesson, plus:

  • 1 small hammer
  • 1 block, soft wood 2 inches x 4 inches x 1 inch 1 wide mouthed bowl or cup
  • 1 pen knife

Comments: Any bowl may be selected for a model, but it should not have handles or undercut portions. It should be absolutely level across the top. Turn the bowl over and look straight down on it. If any portion of the outside surface curves inward so that part of it cannot be seen, that part is undercut, and the bowl will not do for a flood mould. A cereal or similar bowl, in which the base does not flare out, will do quite well.

The object selected for a model, if ceramic, should be glazed all over the outside and bottom. If it is not, the bisque portion must be shellacked. If the object is unfinished wood, it must also be shellacked. If it is glass no further action need be taken, as glass is nonporous. If the object is plastic clay it must be quite moist, and solid clay through and through (a hollow plastic clay bowl would collapse from the weight of the plaster placed on it).

Fig 28

Procedure: Place the 18 inch x 18 inch wooden square on a level table. Place the model, with the cavity facing down, in the centre of the wooden square. As before, wet the 1-inch paint brush and work up a lather with the brown soap. Heavily soap the entire outside surface of the bowl, and the wooden square around the outside of the bowl, with the paint brush (Fig. 28A). Squeeze out the lather from the brush with the fingers. Pass the brush over the bowl and wooden area to remove all suds, leaving only a soap film. Make sure no soap bubbles remain.

Prepare a half basin of plaster of Paris. Spoon plaster on top of the bowl and let the plaster run down on all sides to form a thin layer (Fig. 28B). Spoon more plaster outside and around the bottom edge of the bowl. Build up the plaster around the base to a thickness of 1 inch outward from the model. (Do not spoon more plaster on the model now, since it will run down and spread over the board.) Wait until the plaster around the base begins to stiffen visibly. Now build up the plaster to another layer above the layer around the base, to a thickness of 1 inch. Let this portion stiffen visibly. As each portion stiffens, build up above it until the whole model is covered with plaster to a depth of 1 inch out.

As a principle, in building up plaster around a model by spooning, never spoon soft plaster on top of soft plaster-as both will run down. Wait until the lower layer has visibly stiffened. When spooning on plaster, avoid as much as possible disturbing layers which have already been spooned on the model. Agitation of the plaster tends to pack it at the point where the metal spoon touches it.

With the scraper, smooth the outside surface of the plaster and make a flat area across the top (Fig. 28C). This flat area on the top should be level, so that the mould may eventually rest evenly on it when used for casting. Let the plaster set for about 15 minutes. Check the setting by feeling the warmth of the mould. After 25 minutes, lift the mould upside down. Run the pen knife, along the outside upper edge of the model, to a depth of 1/16 inch, with the sharp edge of the knife facing out, and the smoother side against the model (Fig. 28D).

After the plaster cools, see if the model will come out of the mould easily. If so, remove it, and examine the cavity of the mould.

There should be no air pockets in the plaster. If there are, these can be filled in very carefully with soft plaster-but extreme care must be used to insure that the surface is even and smooth.

If the model does not come out of the plaster easily, lift up the mould, in the hand. Place the wooden block on the shoulder of the plaster mould. Rap the wood block sharply with the hammer (Fig. 28E). This will loosen the model so that it should slide out easily.

Explanation of techniques: The reason for putting a soap film on the model is to prevent the plaster from adhering to the model, when the plaster sets.

The soap bubbles are removed to prevent air pockets from forming in the plaster.

The pen knife is run along the outside edge of the model to remove any lip of plaster which may have run under the rim of the bowl; and which might prevent removal of the bowl from the mould.

Hints: If plastic clay or plasteline has been used for the model, it can be removed by hand or with a knife. Plaster clay and plasteline do not have to be soaped when being prepared for use as a model; however, they undoubtedly will be destroyed or damaged upon removal. Be sure to clean out all clay particles from the mould cavity with a stiff dry hair brush, after the particles are dry.

Special separators to be put on models can be purchased at ceramic supply houses; but fat-content soaps, such as brown laundry soap, are most desirable.

Part IV. Making a press mould

Materials: Same as in part III of this lesson. The bowl is not needed.

Comments: A press mould is a one-piece mould, but the cavity is usually much smaller than that used for slip casting. In a press mould very moist plastic clay is tamped into the cavity by hand or rolled in with the rolling pin, trimmed even with the top surface of the mould, and permitted to dry. When the clay dries sufficiently, the pressed piece shrinks and falls out of the mould. Ceramic buttons or other small ornaments (which have no undercuts in the design) are easily and quickly made by this method.

Plastic clay or plasteline can be used as models. The plastic clay should be quite moist. These materials do not have to be soaped prior to use as models.

Procedure: Place the model face up on the 18 inch x 18 inch wooden square. Soap the wooden surface around the model. Spoon soft plaster over and around the model. Now using the paint scraper, form the plaster in the shape of a cube, or cylinder, with a flat horizontal top above the model, to a thickness of at least 1 inch.

Fig 29

A small open box of shellacked wood may be made from the wood pieces screwed to the wood square, large enough to be at least 1 inch larger in each direction than the model. The clay model is placed in the centre of this open box on the wood square. The plaster may be spooned or poured into this box (Fig. 29A).

When the plaster sets, the model should be permitted to dry, shrink, and fall out of its own accord. Clean the cavity of all particles of clay (Fig. 29B). Sandpaper or scrape the edge of the mould cavity very slightly to round it.

Note: Several designs for ceramic buttons or ornaments are illustrated as examples of what can be done with press moulds (Fig. 29 C, D, E, F).

Lesson IX

Part I. Making a two-piece mould

Materials: Same as lesson VII, plus:

  • 1 flexible steel palette knife
  • 4 pieces of Plexiglas or other plastic nonporous sheet, 1/8 inch or 1/4 inch thick (for size see note below)
  • 5 pounds plastic clay, any colour, well wedged
  • 1 heavy lead pencil
  • 1 strong teaspoon
  • 1 simple glazed model, hole in the base (see text below)

Comments: This lesson indicates the principles to be observed in making a two-piece mould. These principles are applicable to the making of any multi-piece mould.

We shall select a ceramic duck as a model, as being a simple item with which to begin. Any simple model may be used. The model selected should have a smooth glazed surface with a hole in the bottom.

"In the trade," moulds comprising actually three pieces, in which two of the pieces contain the cavity, and a third or bottom piece contains the gate and forms the base of the casting, are often called two-piece moulds. The pieces of Plexiglas will be used to form a hollow box, in which the plaster mould will be formed. Measure the length, width, and height of the model. The pieces of Plexiglas should be larger by 2 inches on each dimension than those of the model, so that there will be a thickness of at least 1 inch of plaster at all points.

Fig 30

Procedure: Lay the model (duck) on its side. Look down upon it from directly overhead. Your eye should be at least 2 feet above the centre of the model. Note the outermost edges or outline of the duck which is visible to your eye. With a pencil, mark this outline on the duck (Fig. 30A). This pencil line will be the dividing line where the two parts of the mould, which will be formed around the duck, come together. This dividing line is in effect the outline of the silhouette of the duck.

On the duck, the dividing line will run from the front point of the base, up the centre of the duck's chest and neck, divide the bill in two, run across the top of the head, down the back of the neck, across the centre of the back, along the outermost edge of the tail feathers, down the centre of the lower rear of the duck, to the base again.

Now study each side of the duck, divided by the line. There must be no undercut surfaces on either side. If there are undercuts, it will be impossible to remove the model from the mould without damage.

The surfaces of the model must curve or fall away on either side of the dividing line. This line does not have to be straight, nor does it necessarily divide the model into two equal parts. In fact, if a model is not absolutely symmetrical (exactly equal and identical on both sides) the dividing line will never be straight throughout.

Stand the duck erect on its base on the table. Now, cover up one side of the duck right up to the dividing line with plastic clay. Do not place any clay on the bottom of the base. Do not cover the dividing line with the clay. Build up the clay until it stands out from the line at least 1 inch and until it forms a block

2 1/2 inches deep, 8 1/2 inches long, and 7 1/2 inches wide with the duck buried in it up to the dividing line. (These dimensions are 1 inch larger in each direction than this particular duck) (Fig. 30B).

Lay the block of clay on a side with the duck uppermost. Trim the clay with the palette knife so as to form a smooth clay shoulder out from the model, horizontally and following the dividing line (Fig. 30B).

Place the pieces of Plexiglas around the block of clay. The base of the duck should be flat against one Plexiglas side. Secure the pieces of Plexiglas against the clay block by tying twine around the outside. Fill in the inside corners of the box formed by the Plexiglas with plastic clay, to even the level of the shoulders.

Apply a soap film all over the exposed part of the model and the clay shoulder as in lesson VIII.

Now prepare a pan of plaster of Paris, as indicated in lesson VIII. Spoon plaster into the corners of the open Plexiglas box, and on top of the model (Fig. 30C). Pour plaster into the Plexiglas box until the model is buried to a depth of 1 inch above the highest part of the model. Level the top of the plaster with the paint scraper. Let the plaster set.

Fig 31

After the plaster hardens, remove the Plexiglas sides. Turn the block of plaster and clay upside down so that the plaster rests on the bottom. Peel away all the plastic clay. Examine the model and shoulders of plaster. The plaster should follow the dividing line exactly. Trim the shoulder of plaster to a smooth surface with a pen knife (Fig. 31A). Fill in any holes in the plaster with soft plaster. The model may be removed from the plaster. Do not dig out the model; tap it out. The shoulder should be gently sandpapered to smoothness. Do not sandpaper the shoulder so that it drops below the level of the dividing line of the model. Replace the model in the plaster indentation.

Note: The first time you make a two-piece mould, you may have some doubt as to where the dividing line should be located exactly. Assume that the line lies more toward the side which you are covering with plastic clay. This will result in more plaster, to be formed on the opposite side-perhaps beyond the depth where the dividing line should be.

After the plaster sets and the clay is peeled off, try to tap out the model. If It fails to come out, you can cut or trim back the plaster shoulder with a pen knife until the model does come out. The second plaster half can be then formed against the first half as indicated below. Remember that you can always cut or trim back the plaster, but you cannot add on plaster if you failed to form enough at first. Thus, it is better to overlap the first piece of plaster and cut it back the proper amount.

With the spoon tip, dig out four key holes in the plaster shoulder, by twisting the spoon back and forth in a circular motion at each point (Fig. 31A).

Clean the surface of the model and plaster shoulder of any remaining bits of clay, with a stiff hair brush. Now, apply a heavy soap film to the model, all over the plaster shoulder, and in the key holes. This soap film is a separator between the two parts of the mould.

Secure the Plexiglas sides around the plaster block again with twine. Spoon and pour plaster of Paris into the space formed by the Plexiglas box above the model, to a depth of at least 1 inch above the highest part of the model (Fig. 31B). Smooth the top surface with the paint scraper. Let the plaster set.

Fig 32

After the second piece of the plaster mould has hardened, remove the Plexiglas sides. Secure the two pieces of plaster together by tying twine around them. Turn the assembly so that the base of the model faces up. With the knife and sandpaper, smooth the top surface of the two pieces of plaster, keeping them as level as possible. With the teaspoon tip, cut two keyholes in each plaster piece (Fig. 32A).

Observe the hole in the base of the duck. Carve a piece of wood, in the shape of a tapered cork, so that it will fit exactly in the hole and taper outward, and extend at least 1 1/2 inches above the top of the hole. With the tip of the knife, pack plastic clay between the wooden plug and hole, so that no plaster can run in between the wooden plug and the wall of the hole in the model (Fig. 32B).

Apply a heavy soap film to the top of the plaster pieces, the key holes, the base of the model, and the wooden plug.

With the spoon, apply plaster to the top of the plaster pieces, and build it up to a depth of at least 1 inch. Plexiglas sides can be extended from the sides of the two plaster pieces, and the plaster spooned and poured in the hollow box so formed, instead of building up the plaster manually, if desired. Let the plaster set (Fig. 32C).

Now with the penknife and sandpaper, round all outside edges and corners of the plaster pieces. Remove the top plaster piece. Remove the wooden plug by tapping with the hammer, if necessary. Slightly round the topside edge of the gate left by the removal of the plug.

Separate or pry open the two other pieces of plaster. Remove the model. Examine the cavity formed in the pieces of plaster. The soap film is now washed out with the moist clean brush. The plaster pieces are set aside to air dry. They are now ready for use as a mould.

If the mould pieces separated comparatively easily, and no plaster was broken off in removing the model, castings made from this mould should be exact replicas of the original model.

Note: The above procedure is followed in making all kinds of "two-piece" moulds.

Explanation of techniques: The third piece is made 1 inch thick to provide absorptive body for the base of the casting.

The hole or "gate" in the third plaster piece always tapers inward to facilitate slip pouring. Spare will be formed in the gate which must later be cut away as indicated in lesson VII.

Imagination must be employed in determining the dividing lines to be drawn on the model. You must visualize how each plaster piece can be withdrawn from the model without damage. The dividing line is the line which will later be the line at which the mould pieces separate.

Ridges or mould marks will form on castings at this dividing line (which must be trimmed from the castings later on). Sometimes it is possible to determine from an original ceramic model where these ridges were, if the model was not trimmed properly; and these are a guide in drawing in your dividing lines.

If you find that your first mould pieces are difficult to separate from the model, do not be discouraged. This happens to even the best of mould makers. Break the plaster piece if necessary, remove the model, and try again. The second time, you will be much more successful.

Part II. Multi-piece moulds

Because of the shape of the model it is often necessary to make moulds of many pieces. Considerable experience in casting and mould making is necessary to figure out the number of pieces needed, and where to put their dividing lines on the model. If you are seriously interested in making multi-piece moulds, visit a ceramic plant and examine the various pieces which make up such a multi-piece mould. Study the model, and you will readily understand why the dividing lines (where the pieces join) were selected as they were. You will note that each piece contains no undercutting, and can be withdrawn from the model without damage to either.

Some very complicated ceramic moulds may require as many as thirty pieces. Sometimes, in larger objects, the separate pieces are further encased in two larger plaster shells, so as to keep the numerous inside pieces in place.

In the procedure in part I, this lesson, plastic clay was initially placed on one side against which the first piece of the plaster mould was formed. This is also done in multi-piece moulds; however, as each plaster piece is made, a portion of the clay for the next piece is removed, the surfaces of the first plaster piece and the remaining clay is soap-filmed, and the second plaster piece made between.

Some ceramists, especially if they are using plasteline models, may insert a thin metal plate along the dividing lines into the model, and build up the plaster piece between these plates instead of covering the remaining portion of the model with plastic clay. The use of these plates is applicable only where these dividing lines are fairly straight.

Mould making is a phase of ceramics demanding a considerable degree of craftsmanship. Commercially, if ceramic reproductions are to be sold at popular prices, the model must be so designed that three or four pieces are all that are required for the mould. Too many pieces make the product quite prohibitive in cost.


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