There are several peculiar problems you are likely to encounter while casting with molds.A mold should be planned so that as shrinkage takes place in the slip, all areas shrink toward one central point.If a piece shrinks in different directions, the strain will cause it to crack and break in the mold.The form which best demon­strates this is that of an egg-cup.When such a shape is cast, the slip will shrink toward two points, one above and one be­low.The strain at the narrow neck in the center of the egg-cup would cause it to snap in two.For this reason, an egg-cup is best cast in two separate molds—one for the upper half and one for the lower half.The two sections are then stuck together with mending slip before firing.

Molds must also be designed so that no air can become entrapped when the slip is poured.If you were to cast an elephant figurine, for example, air would be caught at the tip of the trunk.The form of the animal would be upside down when the mold was ready for casting, and air would be trapped in the trunk because it would have no way of escaping.

Wait approximately 10 or 15 minutes—depending on how wet mold is from previous pourings—and pour out slip into receiving jar, again slowly and steadily.When most of slip is out turn mold upside down on two sticks to drain even further.

When the slip has dried enough to remove top of the mold easily, here is what you will see.This usually takes 10 to 15 minutes.If the mold is not ready to come apart, wait a little longer as forcing off top may damage piece you're molding.

For models with similar dead-ends in the mold, carve a tiny vent into the mold with a knife which will permit the air to escape.A tiny stream of slip will run part of the way into the air vent when the cast is poured.This will break off when the clay cast is removed from the mold, and must be lifted out of the vent before the mold is used again.

Another special problem is presented when you cast an object with handles, such as a vase.If you cast the vase with handles and all, you must remove it from the mold as soon as the clay is strong enough to hold its shape—usually about 10 minutes after the slip has been poured.Otherwise, the handles are apt to snap off.This is a deli­cate procedure, since there is always the danger of distorting a piece by handling it so soon after it has been cast, while it is still in a semi-soft stage.For this reason, many ceramists prefer to cast vase and handles separately, the handles being stuck on later with slip.If any sections of the model are to be joined together—for example, handles on a vase or arms on a figurine—you should do so while the clay is still damp.Use the mending slip on both surfaces to be joined and press firmly to­gether.After a few minutes, remove the excess mending slip with a wooden tool and damp brush for the final smoothing.

Occasionally, very fine holes, called pin-holes, will appear in cast pieces.Some­times you will see them as soon as the ware is taken from the mold.Sometimes they will not show up until the piece is in the finishing process.Sometimes they will not appear until after the ware is fired.

Several things can cause these bother­some pinholes.If they appear on the mold side of the piece, it is more than likely that the mold was too wet when the slip was poured into it.If the holes show up on both the mold side and the inside of the ware, chances are they were caused by air trapped in the slip itself.Air can be trapped in slip if it is agitated too much, if it is strained carelessly, or if the clay in the slip is ground too fine.Pinholes will also mar the outside of a cast piece if the slip is poured too quickly into the mold.Dirt or other foreign matter in the slip, which burns out in the kiln, will cause pinholes to develop during the firing proc­ess.That's why you should always run slip through a 40- to 60-mesh sieve before using it for casting.

After you've opened mold, wait a minute or two and try to remove entire piece with plaster mold blocks still attached to legs.Then allow it to stand an additional three minutes on the blocks, and you will have a perfect piece of greenware.

Below, remove leg blocks and inspect for breaks and cracks.There shouldn't be any; if there are go over break with moist sponge.Then let dry on a plaster bat for about a week, until bone-dry.Bottom photo, ware has dried thoroughly and is gone over with fettling knife to remove the mold marks.Clean all appendages with utmost delicacy.

In order to achieve peak results with any mold, it must be properly cared for.To mend a broken mold: First, the mold should be dry.Apply a liberal amount of orange shellac to the broken surfaces.Light with a match and allow to burn no longer than ten seconds, otherwise the heat will harm the mold.Extinguish the flame and press the broken pieces together, holding them firmly for approximately one minute.Any excess shellac that squeezes out should be cleaned off with a small tool and then wiped with a cloth dipped in alcohol.After drying for a day or two the pieces will stay firmly in place.

Too much stress cannot be placed on the necessity for keeping your molds spot­lessly clean.Foreign matter or rough sur­faces formed by caked slip will only ruin your castings.In cleaning a mold, use only a moist sponge.Any other method may damage the plaster.

Powder the inside of your mold with talc if it does not release cast ware easily.Sprinkle on a thin coat.The powder will show on the clay, but can easily be sponged off.Keep your molds in a dry place at room temperature.Dampness encourages fungi to grow on the plaster.By following these few simple rules, you will get many years of use from your molds.

When major cleaning-up is finished it is a good idea to go over areas where marks were with dry composition sponge, not rubbing too hard.Then use slightly dampened sponge, which will remove clay dust and provide good paint-holding surface.

Finishing

The pieces cast in your mold, after they are completely dried, are called greenware.Before this ware is decorated and glazed for the kiln, it must be smoothed, or finished.Unevenness, mold seams, finger­prints and other irregularities must be removed.

If a piece of greenware has been properly dried, this finishing process need not be difficult.Always strive for slow and uni­form temperature when an object is re­moved from the kiln.A drafty, cold room might cause the ware to dry faster on one side than the other and make it warp or crack.The dangers of firing a piece which has not had all the moisture removed have already been discussed.

To finish a piece of greenware, you will need one or more of the following: a fet­tling knife, a piece of fine sandpaper, a sponge, and a pan of water.Handle the piece with care.It is fragile and too much pressure might cause it to crumble in your hand.

First, you should fettle away the mold marks.As a mold gets older, the space between various sections of the mold becomes larger, and the seams get corre­spondingly bigger.When the seams become too heavy, you should discard the old mold and make a new one if further casts of the piece are desired.

Never attempt to scrape the seams off a piece that is still damp—the knife marks will show through the glaze in the finished ware.Carefully take the dry greenware and hold the cutting edge of the fettling knife against the ridge left by the mold.Gently scrape it down until it is flush with the surface of the ware.Then carefully sand the seams with a fine sandpaper.

This will cause a clay dust to form on the surface.You can remove this dust with a damp sponge.Make certain that the sponge is not too moist.If it is difficult to sponge in some places—such as the inside of a small pitcher—you can use a stiff-bristled brush instead.If too much water is on the sponge or brush, it will wash away some of the clay, leaving the silica in the clay exposed.This is undesirable because the silica will make the surface of the ware rough.

The technique for mending cracks has already been discussed in the chapter on clays.To review briefly, dampen the area around the crack with a damp sponge.Probe to the bottom of the crack and fill with mending slip.•

It was decided to paint this piece as an English or Springer Spaniel, and this called for black on white.Do not be reluctant to go off beaten 'track in your painting; main thing is not to get duplication of live animal but a true "ceramic."

Piece is covered with transparent glaze, flowed on with uniform thickness.Flow glaze into leg openings to strengthen them.Dry-foot the ware.Inspect glaze for missed spots, easy to see as glaze dries.When piece is dry.place it in kiln.

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