Numismatic and History Discussion Forums > Coin Photography, Conservation and Storage

Making plaster casts of coins

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curtislclay:
Ingredients:

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.

Procedure:

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 or aureus, a larger ball for a middle bronze, still larger ones for sestertii and medallions.

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 batch!

To avoid bubbles on extremely sharp coins, usually FDC aurei or denarii: 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.

Jerome Holderman:
Thanks Curtis! I have been wondering about the correct procedure for this for some time.

Is making 2 sided casts more trouble than it is worth? I'm sure getting the alignment just right would be tricky, and the slightly thicker plaster that may be required, would be more susceptable to air entrapment.....

I might just give it a go anyway, just out of curiosity...

curtislclay:
Jerome,

Mionnet's sulphur casts, and the sulphur casts made to illustrate Capt. Smyth's book of his "Roman Brass Medals", are two sided, just like the real coins.  I don't know how they were produced.

I have never seen a two-sided plaster cast.  However, having separate casts for obv. and rev. is enormously more convenient for study,

(1) because you can see both sides at a glance, without having to turn the coin over. 

(2)  Because, having identified and catalogued the obv. dies, you can leave them lying in order by dies, but remove the reverses to arrange them too by dies, but separately.  Say a new spec. with Pax rev. turns up and you want to identify its rev. die.  How much more convenient, less prone to error, and less time consuming, to have to consult just one tray of Pax rev. dies, grouped by die, than to have to examine every coin of the issue, picking out the Pax coins from among the other rev. types, if the reverses remain attached to their obverses which are arranged by die!

(3)  Because you can write the necessary info about provenance, weight, die axis, die number, on the back of the casts themselves.  The same info written on a coin ticket to accompany a two-sided cast, is much more likely to get attached to the wrong coin, or simply be lost!

So no one interested in studying the coins would have the slightest desire to make two-sided casts!  They would make it fully twice as hard, I would say, to examine and comprehend the material.

Robert_Brenchley:
Not only that, but they're a lot harder to make. You'd have to make the first cast as described, then cover the surface with vaseline or equivalent to stop the two halves sticking together. Then do the second half on top. I would imagine that sulphur casts would be done with plaster rather than plasticene negative moulds, given the temperature of the stuff. You'd need a matchstick or something to cast a hole for pouring. That's not a good description, but you can see the complications. I don't think plaster would work for two-sided casts; you'd need something like wax or sulphur which poured really well, and took a good surface. If you google 'Lost Wax Casting' you'll probably find something which could be adapted.

Jerome Holderman:
Thanks guys,

Sounds like it is probably not a great idea. I was thinking about it more from a perspective of making them to give away to students in my sons class, if I do a presentation there next year as planned. I intend to give out some LRBC, but thought they might enjoy casts of some more expensive or less common types. I still may give it a try for that purpose, just to see if it could be done?  ::)

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