Plasticine. I used to get modeling clay for kids in toy stores in England
, but now get modeling clay for artists in art supply shops. Roma
Plastilina is an Italian
brand widely available in the US. This is NOT the clay that turns hard when you bake it! It is modeling clay that always remains soft, though gradually it gets firmer as it absorbs talcum powder. Eventually it has to be replaced when you can no longer press a sestertius
into it without breaking your thumbs!
Talcum powder, ideally unscented. Available in any pharmacy, used for babies' bottoms!
Plaster of Paris, available in art supply shops, also dental/medical supply shops, since dentists and doctors use it too.
A small bottle
of mineralized, methylated spirits, also called completely denatured alcohol. I got mine at the chemist Boots in GB, and have never bought it in America.Cup
; teaspoon; pocketknife; small, soft, water-color paintbrush; running water.
1) Making impressions.
Knead a small wad of plasticine, or the plasticine impression of a coin that you have already cast
off, to make it soft and pliable. Roll it into a small ball, and flatten it into a coin-sized disk between two pieces of paper with a book on a tabletop. Naturally you need only a small plasticine ball for a denarius
, a larger ball for a middle bronze, still larger ones for sestertii
Apply talcum powder to the upper surface of each of your blanks, otherwise the coin will stick in the plasticine and you won't be able to remove it without ruining the impression!
Press the coin into the powdered disks, one disk for the obv. and a second for the reverse
. Sink the coin 2-4 mm into the plasticine if possible, so the plaster cast
you end up with will be thick enough to be durable.
To remove the coin after making each impression, gently pull back the plasticine from one edge until the coin begins to come free. Then turn the impression upside down and remove the coin fully with your fingernail and the force of gravity, or by momentum, holding the disk upside down about half an inch (1 cm.) from your fingertips, and banging your fingertips against a piece of felt or other soft surface on your table.
Reflatten the plasticine disk, which will have become somewhat bent and convex through removal of the coin. Throw the flat side of the disk hard against the tabletop, and with your fingertips pull the upper outer edges of the impression lightly inwards and upwards. You want your cast
to be flat like the coin, not saucer-shaped!
2) Casting off the impressions.
In my experience between about 20 and 36 impressions (representing 10-18 coins) can be cast
off at one time. After that the plaster has become too thick and viscous to flow into the details of the impressions. This will vary with the setting speed of the plaster you are using, and your own working speed.
Lay out the impressions in rows on a piece of paper on the table.
Fill the cup
half full with cold water. With the spoon, sprinkle plaster of Paris into the water, allowing it to absorb water as it sinks to the bottom. Do not stir. Three heaping teaspoons of plaster should suffice for about a dozen denarii
, five heaping teaspoons for about a dozen sestertii
or middle bronzes.
Pour off almost all of the excess water above the plaster, leaving only a layer perhaps 2 mm deep. Now, not earlier, stir with the spoon! This procedure easily gives you the right consistency of the plaster and water mix, which should be like thick cream, not too thin like water nor
too thick like honey or peanut butter. If the mixture seems too thick after stirring, add a little water and restir.
With the spoon, pour plaster into each of the impressions, starting near the edge of each impression and letting the plaster flow across the surface, in order not to entrap air bubbles. Add plaster until it protrudes above the surface of the impression, without allowing it to flow over the edge onto the paper, though this inevitably happens to me a couple of times per
To avoid bubbles on extremely sharp coins, usually FDC aurei
: using the paintbrush, cover the surface of the impression with mineralized spirits before pouring in the plaster. Make sure the alcohol goes right to the bottom of every letter of the legend
and every detail of the type
. Since the alcohol evaporates quickly, you have to pretty much alternate applying the alcohol and pouring the plaster, impression by impression, or at most apply the alcohol to two impressions, pour the plaster, apply alcohol to the next two impressions, pour the plaster, and so on. I find that the alcohol is not usually required to produce bubble-free casts of bronze coins however sharp, and of somewhat worn gold and silver coins.
Allow the plaster to dry and harden for an hour or more. The casts you poured last will harden quickest, because by then the plaster mixture had
already gotten much thicker.
Remove the hardened casts from the molds. Bend back the plasticine a little, and lift the cast
out by inserting your fingernail under the protruding edge of the cast
. If you are careful, you will not damage the mold and can use it to make duplicate or triplicate casts if required.
With the penknife, trim off most of the protruding edge of the casts and flatten their backs. If the cast
is thicker on one edge than the other, try to even out the thickness, so the surface will be level when the cast
is lying in a drawer or is glued to a glass
plate for photography. This trimming is easiest to do when the plaster has set enough for the cast
to be removed from the mold, but has not yet completely dried out. A day later, trimming will take more effort, and will produce bothersome clouds of plaster dust!
Place the trimmed casts on a piece of newspaper and allow them to dry completely, usually 24-36 hours, less if you put them on a radiator or in a drying cupboard. You can now write pertinent details on their backs with pencil: source, weight and axis
, die number, etc.
CAUTIONS: Bronze coins with fragile patinas. The plasticine can pull off bits of the patina
! Use extra talcum powder, or forego casting altogether for particularly endangered coins.Crystallized
or particularly thin silver coins. Such coins may BREAK when you try to press them into the plasticine! Use only very soft plasticine and press the coins very gently, or forego casting altogether.