Slip casting ceramic objets

Slip casting is one of the simplest methods of reproducing ceramic objects. It has the advantage of permitting the making of a large number of exact replicas of an original model-such as identical sets of plates, ash trays, vases, statuettes, bowls, and others-with a minimum of effort. With slip casting it is possible to cast duplicates of most models which are made by the plastic method.

To acquire a better knowledge of slip casting, in the next two lessons we will deal first with the making of casting slip and then with the making of clay castings from moulds.

Casting slip is made from clay in dry form, water glass (sodium silicate), and water in proper proportions and of the proper consistency.

To understand better why slip must be of the proper consistency, the employment of slip in casting is explained here. All moulds for slip casting are made of plaster of Paris. When slip is "poured" into the cavity of the mould, the plaster of Paris immediately begins to absorb water out of the slip. This causes clay to form and thicken where it is in contact with the plaster walls. When the clay has formed to the proper thickness, the remaining slip in the centre of the mould cavity is "drained" off. The clay in contact with the plaster remains and is permitted to stay until it dries sufficiently to attain a "leather" consistency. At this point the clay casting will separate easily from the plaster surface and can be removed from the mould.

If there is not sufficient water in the slip when it is first made, the pouring of the slip into the mould will be uneven, and the clay will form unevenly against the inside surfaces of the mould. The clay remaining after draining the surplus slip will be of varying density and will warp when fired.

If there is too much water in the slip when it is made, the drying time of the slip in the mould will be unduly prolonged, the slip will have a tendency to coagulate, and will not drain easily. Also, the remaining clay, after drying to leather hardness will not separate easily from the mould.

A casting slip can be made from clay and water alone. But such slip requires much more water; furthermore, the clay will settle to the bottom of the container in a short time, and must be re-stirred each time this slip is used.
The addition of water glass, or a "wetting agent" as it is often called, necessitates less water for the mixture. In slip with water glass the clay will not settle out, can be stored in a sealed or lidded container for long periods, and is ready for use with little re-stirring.

Remember, however, that the plaster must absorb water from the slip. If the slip can be made of proper consistency with less water, clay will form more quickly on the cavity walls, the excess slip can be drained earlier, and the remaining clay will arrive at leather hardness sooner.

Numerous types of casting slip already prepared can be purchased commercially. However, if the student intends to make numerous castings, it is most desirable, from a practicable and economic viewpoint, that he learn to make his own casting slip. With little practice he can learn to mix slip and determine when it is of the proper consistency.

As a matter of general information, all clays in the natural state ("raw clays") contain varying amounts of mineral salts including sodium silicate (water glass) . However, if the clay has weathered in the open for a long period of time, rain, snow, and frost will tend to wash out some of these salts. Firms selling dry clay will usually specify the proportion of clay, water, and water glass to use for making casting slip with their clays; such formulas are based on the amount of salts existing or remaining in their clays.

In purchasing dry clay, specify that the clay is intended for making casting slip, and be sure to ask for the proportions of clay, water glass, and water required for that particular clay. The proportions are usually given by percentage of weight; so, when measuring the amounts of each ingredient, weigh them on a scale. It is most important that the amount of water glass be correctly calculated, because a few drops too much added to the mixture may make the consistency of the slip unsatisfactory.

With a little practice and care, proper consistency of slip is easily determined and should cause little difficulty.

Lesson VI.

Preparing slip to proper consistency

Materials:

  • 100 pounds of clay, dry form, low firing, any colour
  • 2 pounds of water glass, liquid
  • 1 pointed stick (the handle of an artist's paint brush will do nicely)
  • 2 five-gallon lidded containers of wood, earthenware, or galvanized iron 1 broom handle or long stick
  • 1 large size flour or tea strainer
  • 1 scale (indicating pounds and ounces)

Comments: For the purposes of this lesson let us assume that the proportion of ingredients, furnished by the ceramic supply house for making casting slip from the clay we have purchased, is:

  • 67% dry clay
  • 32 1/2 % (approximately) water
  • .66 % water glass

Note: This percentage is by weight. We can safely assume that water weighs 1 pound per pint. Water glass is measured in ounces by weight (not fluid ounces). Clay is measured in pounds by dry weight.

Initially we shall make a mixture of slip using 25 pounds of dry clay. By simple arithmetic we calculate that we will need roughly about 12 pounds of water, or 12 pints; and .25 pound of water glass, or 4 ounces.

Procedures: In making slip always begin with about 70% of the total amount of water you calculate you will finally use. In this example, put 8 pints of water in one container. Stir the water with the broom handle in one hand while slowly crumbling and adding clay to the water with the other hand. It is important that all particles of clay get wet; so be sure that all lumps are broken up, and the clay is like powder when added. The mixture should be stirred continuously during the succeeding process. Stir and continue to add powdered clay until the mixture becomes very thick and sluggish. At this point add 2 ounces ( 1/2 the total final amount) of water glass slowly. The mixture will begin to thin almost immediately. Now add more clay until the mixture again begins to become sluggish. Add about two pints of water slowly, stirring meanwhile. The mixture will again thin. After a few minutes of stirring add more clay until the mixture again becomes sluggish. Now add an ounce of water glass to the mixture-drop by drop. The mixture will again thin. Add more clay now until the mixture has a consistency of thick cream. Add a small amount of clay and when it again tends to become sluggish add a little water. Again add clay, and when again the mixture tends to become sluggish add a drop or two of water glass. At this stage it is essential that not too much water glass be added. The procedure is to add small amounts of clay, then a little water, then clay, then a drop of water glass; and repeat this sequence until the 25 pounds of clay are in the mixture. The mixture should have the consistency of heavy cream. When adding clay at each step, do not add so much that the mixture becomes too sluggish for easy stirring. The mixture should be tested by Test No. 1 frequently, and by Test No. 2 when you believe you have the proper consistency.

Note: Never mix the entire amount of water, clay, and water glass all at once, as it will take considerable effort and much more time to arrive at the proper consistency. For such mixing a mechanical mixing machine would be required.

Fig 20

When the mixture tests at the proper consistency, pour it through a strainer into the second container. The strained mixture should remain in the second container for 72 hours to "mature," and then be stirred thoroughly before using.

  • Test No. 1-Insert the pointed stick halfway into the slip mixture, withdraw it, and hold vertically above the mixture. The slip should flow off from the stick point, smoothly, evenly, like new automobile crankcase oil (Fig. 20A). If the slip flows off unevenly in lumps or in large blobs, a small amount of water should be added to the mixture and the whole thoroughly stirred. The test should then be repeated.
  • Test No. 2-Dip the pointed stick into the mixture and permit several large drops of slip to fall on the clean dry plaster bat. At first these globules will glisten with a shiny surface. After 1 or 2 minutes the surface of the globules will turn to a satin finish as the plaster absorbs water from the globules. At this point, push the individual globules with the finger-tip. The globule should roll away from the plaster easily and form into a soft ball (Fig. 20B). If this happens, the slip is of the proper consistency. If, however, the globule sticks to the finger and plaster, and smears across the surface of the plaster, there is too much water in the slip. When this occurs add a small amount of dry clay and stir it into the mixture.

Then test the mixture again with both tests.

Lesson VII

Part I. Making a casting from a one-piece mould materials:

  • A container holding casting slip of proper consistency
  • A dipper
  • A pen knife
  • A breadboard
  • 2 sticks of wood, each 1 foot by 1 inch by 1/4 inch thickness A large strainer
  • A two-quart common pitcher
  • A piece of triple zero sandpaper
  • A small rubber sponge

Comments: A one-piece mould is often called a "flood" mould. Two-piece moulds are discussed in Part II of this lesson. We use the flood mould in this lesson as the simplest means of illustrating the basic process of slip casting.

One-piece moulds are used for casting open shapes, such as wide mouth bowls. For the purposes of this lesson a one-piece mould, preferably a cereal bowl, should be purchased. These are available at any ceramic supply house. Be sure, when purchasing, to specify that you want a flood, or one-piece, mould.

As additional information, low firing clay when drying shrinks about one-eighth in all its dimensions. This over-all shrinkage is of advantage in casting because it tends to separate the casting from its mould as it dries.

Procedure: With the dipper, dip up slip from the container and pour it through the large strainer into the pitcher until the pitcher is almost full. The pitcher should always be sufficiently large to hold enough slip to fill the mould completely at one pouring.

Fig 21

Place the flood mould on a level table with the cavity of the mould face up (Fig. 21A). In pouring, the steady smooth stream of slip is moved above the cavity along the inner edge of the cavity opening in a slow circular motion so as to cover all parts of the cavity at the outset (Fig. 21B). Never pour the slip in one spot in the cavity. Do not let the slip bubble or splash. Pour the slip slowly from the pitcher into the cavity. Pour until the cavity is completely filled and overflows part way on the shoulders of the mould (Fig. 21C). The slip will settle in a few minutes as water is absorbed by the plaster of the mould. Pour more slip until the cavity is again filled to the top. Repeat this filling every 5 minutes or so until there is no appreciable settling of the slip.

Allow the slip to remain in the cavity 10 minutes,. Now test the thickness of the forming clay by cutting a small section of the clay along the edge of the mould with the blade of the pen knife held flat on the shoulder of the mould (Fig. 21D); or the mould may be tilted, and the thickness of the clay estimated by eye. When the clay has formed to a thickness of 1/4 inch, it is time to "drain" off the remaining slip.
In draining slip, tilt the mould over the container and permit the slip to run out slowly. Never drain slip quickly. Drain the excess slip, tilting the mould until the mould is held upside down over the container (Fig. 21E).

Fig 22

Now place the two sticks flatwise on the breadboard parallel to each other and sufficiently far apart so that the clay on the shoulders of the mould does not touch the sticks when the mould is placed upside down on top of the sticks. Allow the mould to rest on the sticks and breadboard in the upside down position for 10 minutes. The balance of the slip will drip out of the mould onto the breadboard (Fig. 22A).
After the 10 minutes are up, lift the mould and turn it face up again. With the flat blade of the pen knife resting on the shoulder of the mould, trim the clay casting even with the top, removing the excess clay on the shoulder (Fig. 22B).

Let the clay remain in the flood mould for 30 minutes or more.

You will note by visual inspection, along the outside edge of the top of the casting, that the clay is separating from the mould. Let the clay remain in the mould until this separation is noted. Now place the breadboard on the mould, turn the mould and breadboard upside down (Fig. 22C). Lift the mould up. The casting should remain on the breadboard. If the casting sticks in the mould, place the spread left hand under the upside down mould. Rap the top of the mould sharply with the palm of the right hand. The casting should fall into the left hand.

Gently place the casting top side up on the table and do not touch or disturb it for 30 minutes. It is now in the leather condition, or "leather hard." Examine the surface of the casting. If there are any air pockets or holes, fill them in with soft clay. Smooth above the filled-in pockets with the damp rubber sponge.

Place the casting away from an air draft or direct sunlight, to dry at room temperature. When the casting is "bone-dry," run sandpaper around the lip of the bowl to round the edge (Fig. 22D). If there are any imperfections on the surface of the casting these can be smoothed by sanding. The casting is now ready for bisque firing.

The mould is thoroughly cleaned of all clay particles after the casting is removed and is set out to dry. The mould must be absolutely dry before being used for another casting.

Explanation of Techniques: Slip is poured slowly and smoothly into the cavity of the mould along the edge to remove all air from the cavity and prevent at the same time the formation of air pockets or bubbles in the clay. In pouring, the stream of slip is moved around the mould, because if it strikes only in one place it will cause the clay to pack at that place. This will result in warping of the casting when bisque fired, in the glaze not "taking" at that place, and in uneven glaze colouring upon firing.

Excess slip is drained slowly and evenly from the mould after clay has formed to the proper thickness. If the slip is drained quickly, the suction of the outgoing slip may pull the soft clay away from contact with the plaster wall in the cavity, resulting in a warped casting.

A small amount of slip is permitted to overflow on the shoulders of the mould when first pouring slip in the cavity. This is done so that the clay will be held in the cavity, and will not drop out when the mould is first turned upside down. This overflowing permits an even trimming of the top of the casting and insures that the top will be exactly level. This overflowing on the shoulders of the mould is done only with flood moulds, and does not apply to two piece or multi-piece moulds.

Each time a casting is made, the plaster of the mould absorbs water from the slip. It is necessary that this moisture dry out of the mould before using the mould again.

"Leather hard" is the condition of the clay when the casting is ready to be separated from the mould. The clay in this condition is moist and somewhat plastic, but able to support itself without collapsing. It is a little less plastic than wedged plastic clay but plastic clay items or ornaments can be slip pasted to the leather hard casting with assurance.

After removal of the leather hard casting from the mould, it can be formed with the palms of the hand to an oval shape top if desired.

Part II. Making a casting from a two-piece mould materials:

The same materials as in part I of this lesson plus: A two-piece mould (see text below)

  • 2 small wooden wedges
  • A ball of cord or twine

Comments: Two-piece or multi-pieced moulds consist of two or more pieces of plaster of Paris which, when fitted together, form a cavity on the inside, which is the shape of the desired casting. The purpose of making the mould in two or more pieces is to permit certain shaped castings, when leather hard, to be removed from the mould. For example, it is obvious that if a wide-bellied, narrow-necked vase were cast in a one-piece mould, it would be impossible to remove the casting without breaking the mould. In a two-piece mould made in two halves, each half can be removed, leaving the vase casting and the mould pieces intact. A multi-piece mould (usually described by the number of its pieces-for example, three-piece mould, four-piece mould, etc.) is designed so that the several mould pieces can be removed without damaging the casting.

Note: The lesson on mould making later on will clarify the employment of the several mould pieces.

Moulds of two or more pieces contain rounded indentations on one face, with corresponding protuberances on the opposite face of the adjacent piece. These are called "keys." When the adjacent pieces are fitted together the keys insure that they will fit in the correct place and arrangement; that the cavity, which the pieces form, is correct; and that the pieces will not shift when slip is poured into the cavity. When the pieces are fitted together for casting they are tied on the outside with cords to keep them secure; and little wooden wedges are slipped under the cords to insure a tight hold. An opening from the cavity to the outside, a "gate," will always be present, through which slip can be poured into the cavity, the air may escape, and the excess slip may be drained.

Fig 23

For this lesson, purchase a two-piece mould, of any available design. A small vase or pitcher with the "gate" at the top is preferable (Fig. 23A).
The length of time the clay should remain in a mould to become leather hard, after the excess slip is drained. away, will vary. This time depends on the air temperature, the humidity, the amount of water in the slip, and the type and thickness of the plaster mould. The best way of judging is to note an appreciable separation of the clay from the mould, at the top of the gate of the mould; at that time the wedges can be removed, the cords untied, and the mould pieces separated.

Procedure: Fit the mould pieces together. Be sure that the adjoining surfaces, the shoulders, and cavity of the mould are perfectly clean and free from particles of clay. The keys indicate the location in which the pieces fit properly. Tie the assembled pieces on the outside with cords, one near the top, and one near the bottom. Secure the cords with bow knots. Slip wedges of wood under the upper and lower cords until they are quite tight (Fig. 23B).

Fig 24

Stand the mould so that the "gate" is facing upward. Pour strained casting slip slowly in against the sides of the gate (Fig. 24A). Avoid pouring in one place, creating bubbles, or splashing the slip. Do not block the gate with inflowing slip. Remember, the air in the cavity must escape as the slip pours in and must have an avenue out; otherwise it will cause air pockets in the casting. Fill the mould to the top. Do not let the slip run onto the shoulders of the mould. The slip will settle. After 1 minute, add more slip. Keep adding slip until there is no appreciable settling of the slip. After 10 to 15 minutes, test the clay for thickness, by tilting the mould or shaving a sliver of clay at the top of the gate with the pen knife (Fig. 24B) . When the clay is 1/4 inch thick, gently tilt the mould and slowly drain out all the excess slip into the container (Fig. 24C). Make sure that the outflowing slip does not block the gate. Turn the mould upside down and rest on the shoulders on the two parallel sticks (Fig. 24D) . After 10 minutes turn the mould straight up again. Let the mould rest in this upright position for 30 or more minutes.

Examine the line between the clay and the mould at the top of the gate. There will be a shrinking and separating away of the clay from the mould. If such shrinkage and separation do not occur, let the mould rest for 10 to 20 minutes more. When there is definite separation, the casting may be removed.

Fig 25

Lay the mould on a side. Remove the wooden wedges. Untie the cords. Very gently tap the upper mould piece. Lift it up and away. If the upper mould piece does not come away easily, insert a knife blade between the two mould pieces and pry them apart gently (Fig. 24E). While the casting is resting in the lower half of the mould, with a sharp knife, cut around the casting where the clay which has formed in the neck of the gate (called the "spare") joins the main design of the casting (Fig. 25A) and remove the spare. Now see if the casting will lift out by hand from the lower mould piece. If it does not, gently tap the lower mould piece on the side. Lift out the casting or drop it into the hand by turning the lower mould piece upside down, and set the casting vertically on the breadboard.

There will be mould ridges, seams or "marks" on the sides of the casting where the pieces of the mould were joined. After the removed casting has dried 30 minutes, the mould marks are trimmed with the sharp knife blade until even with the surface of the casting (Fig. 25B). If there are any air holes or pockets, these are filled in by hand with soft clay and rubbed over with a moist (not wet) sponge. Set the casting away, out of an air draft or direct sunlight, until "bone-dry." When "bone-dry," gently sandpaper the edge of the casting (where the "spare" was cut away) to round it. Sandpaper gently any pimples or other defects on the casting and remove them to give a smooth surface. Handle the casting with care at all times, as it is quite fragile and may mark or crumble.

After the casting is "bone-dry" it may be further smoothed by gently scraping with the knife. The clay will come away as a powder when scraped. Clean the mould pieces and set them out to dry before using again.

Part III. Hints in the slip casting technique

Statuettes and similar objects are usually cast upside down; in other words, when the excess slip is drained from the mould, the hole remaining in the casting is in the base of the final casting, and will be on the bottom. The spare which is formed in the gate of the mould will join the base of the casting and should be cut away. Trim this spare off with the pen knife, while the casting is in the lower part of the mould, after the upper piece has been lifted away.

There will always be mould ridges on every casting made in a two or more piece mould. The quality of the final object will depend on how well these ridges are trimmed and the marks removed.

If it is desired to attach a flower ornament to a casting, the ornament should be in plastic condition (more moist than the casting) and the casting should be in leather condition. They can then be slip-pasted together. Handles for vases or pitchers may be cut out as described in the lesson on slab method of plastic clay work, then shaped and slip-pasted on the leather hard casting. These handles should be more moist than the casting. Use plenty of slip.

While some castings are in leather condition they can be shaped to some degree by hand, to form other shapes.


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