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FORVM`s Classical Numismatics Discussion Board  |  Numismatic and History Discussions  |  Coin Photography, Conservation and Storage (Moderator: bruce61813)  |  Topic: Making plaster casts of coins 0 Members and 1 Guest are viewing this topic. « previous next »
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curtislclay
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« on: July 19, 2006, 04:47:18 pm »

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.







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Curtis Clay
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« Reply #1 on: July 19, 2006, 05:33:43 pm »

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...
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curtislclay
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« Reply #2 on: July 20, 2006, 12:33:37 am »

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.
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« Reply #3 on: July 20, 2006, 01:21:19 am »

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.
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Jerome Holderman
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« Reply #4 on: July 20, 2006, 04:21:42 am »

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?  Roll Eyes
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bruce61813
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« Reply #5 on: July 21, 2006, 03:02:48 pm »

JD, if you can find it is a hardware store, try Rock Hard Water Putty, it mixes with water like plaser of paris, but is brown/tan in color, and as the name implies, rock hard when it sets. It is less brittle and has a fine grain that hold very sharp detail.

Bruce
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« Reply #6 on: August 13, 2006, 06:44:05 pm »

I went to an arts and crafts store this weekend and bought the clay and plaster.  I have not been able to stop playing since I got home.  At this rate I will have 1000 casts before long.

Curtis,  the instructions are flawless and the process is simple and fun. 

I am in between cameras so I can't post pics just yet but they look so cool!!!!!

Thanks Curtis:)

Jeremy
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« Reply #7 on: August 31, 2006, 11:19:12 pm »

If you are serious about doing some two-sided casts, research RTV rubber molds and Alumalite resin. Scale modelers use this stuff (I have cast very fine detailed tiny parts) and there are ample how-to sites out there on the web. I would recommend the squash casting method - two part mold with alignment pegs cast into the mold, fill each side with resin and place together, effectively squeezing the excess into a paper thin flash. Resin hardens chemically in minutes and can be painted and weathered realistically, and as it is plastic won't be mistaken for the real thing.

I may do some myself, but I have to obtain worthy coins first!  Wink
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« Reply #8 on: September 01, 2006, 07:13:15 am »

Here are some links about casting in resin. The quality of the resulting duplicates on a few of the websites is not that good but the method is the same (use care and don't be sloppy, RTV replicates the most miniscule detail)

http://www.largescalecentral.com/articles/view.php?id=42

http://www.freemansupply.com/moldmaking.htm

A more involved mold how-to posted on this board, but shows what I mean by alignment pegs: http://www.hobbyfanatics.com/index.php?showtopic=5150&pid=69562&mode=threaded&show=&st=&

These guys sell a starter kit that is reasonable, if you are frugal with materials the kit should allow you to cast many coins. I have used their RTV formula and their resin before, both are excellent. Resins must be properly measured and well mixed or they will either be too oily when hard or they will not cure fully and will be soft. This pour-a-cast stuff is especially prone to being soft if not measured properly, but when done right it is a wonderful resin, bright white. http://www.bare-metal.com/starterkit_files/starter_kit.html

Alumilite is the other resin I prefer, in fact I like it best but my local supplier sells Pour-A-Kast so I buy it because of convenience. Alumilite is the best quality resin commonly available. On their site they have a wonderful series of how-to articles including one part molds, squish casting and others: http://www.alumilite.com/index.php?page=show_how
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« Reply #9 on: September 01, 2006, 05:54:23 pm »

Here is a picture of my first attempt at casting.  I need to work on it more but it is a whole lot if fun and kind of neat. 

Jeremy
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curtislclay
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« Reply #10 on: September 01, 2006, 06:16:19 pm »

Bravo, Jeremy!  You're an accomplished cast maker already.
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« Reply #11 on: January 18, 2009, 06:09:38 pm »

My daughter had some plaster she got for Christmas and she wanted to learn how to make something interesting (our house doesn't need any plaster toadstools or the other types of stough that is peddled to kids as art).   I found this FORVM thread on a Goggle search which is funny, me being a member here.  I bought some modeling clay and we used baby powder for the talc (that is the major constituent).  I was surprised just how easy it was to do.  The pictures show our first attempt.  The method of spooning the plaster powder into a cup of cold water and then draining the water off made exactly the constutuency described (and it did thicken very quicly with time).  It took about fifteen minutes to have the dies made and plaster poured, and then we pulled out the casts an hour later.  Thanks for the great instructions Curtis!
Richard
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« Reply #12 on: May 15, 2009, 07:33:24 pm »

Has anyone ever tried to gild a plaster cast?  I keep my gold coins in my safe.  I would like to try to make some gilded plaster casts that I can enjoy while the coins are safely locked away. 
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jamesicus
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« Reply #13 on: May 15, 2009, 09:09:59 pm »

Has anyone ever tried to gild a plaster cast?  I keep my gold coins in my safe.  I would like to try to make some gilded plaster casts that I can enjoy while the coins are safely locked away. 
Certainly plaster casts are invaluable for the study of rare coins as the following attests. I am not really sure of my description of "galvano" copies -- I hope Curtis (and maybe others) can explain the process better -- I am anxious to get it right for my web page.

Ten aurei multiple/medallion commemorating the restoration of  Britain to the Roman Empire (Arras Hoard) -- RIC Vol. VI, Treveri - No. 34 -- (bronze?) copy depicted here:


FL VAL CONSTA -- NTIVS NOBIL CAES ......................... R -- EDDITOR LVCIS -- AETERNA -- E
   
This historically important coin resides in the museum at Arras. Bastien records it as  No. 218 in his book on the Arras Hoard and mentions that galvano* copies were made and sold by the Paris coin dealer Bourgey. Edit: I thought my specimen to be one of those Bourgey copies, however at a weight of 30.1 grams it may be a cast bronze replica from a galvano  copy(?).

* galvano: a plaster cast of an original coin electroplated with metal.

James

Edited to delete extraneous material.
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« Reply #14 on: May 16, 2009, 09:12:50 am »

Is there a way to harden or seal the plaster after it is dried or can you handle them at random without worrying about losing detail or darkening them with finger oils?

Chris
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« Reply #15 on: May 17, 2009, 07:36:39 pm »

This weekend I experimented with gilding some plaster casts with gold leaf.  I used an oil-based gilding method, which is much simpler and faster than water gilding, but the results are never quite as good. 

The results were decent -- about what I was expecting.  I will post some pictures in a few days (I am still experimenting and I want to see if I can improve!).   

It is a coincidence that casata137ec raised the question about seals.  In order to gild anything, you need to seal the plaster with a skin.  The gold sticks to the size,  the size sticks to the skin, and the skin sticks to the plaster.  If you put the size directly on the plaster, it just soaks into the plaster like a sponge and never gets sticky.

In my experiments I created skins using a variety of techniques.  I tried diluted acrylic paints, diluted oil paints, diluted oil paints mixed with a medium, and just plain Liquin.  The best results from my first batch of gilded coins was from the Liquin.

If you are not gilding and you just want something that looks good, the acrylic paints were great.  It is very easy to dilute them, and they dry very quickly.  That means you can put on several ultrathin layers in less than 30 minutes.  The layers are so thin that they do not affect the details.  The details become much more visible with the paint compared to white plaster.  I will include some pictures of painted coins when I make my next batch . . . .
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« Reply #16 on: May 17, 2009, 09:44:44 pm »

This weekend I experimented with gilding some plaster casts with gold leaf.  I used an oil-based gilding method, which is much simpler and faster than water gilding, but the results are never quite as good. 

The results were decent -- about what I was expecting.  I will post some pictures in a few days (I am still experimenting and I want to see if I can improve!).   

It is a coincidence that casata137ec raised the question about seals.  In order to gild anything, you need to seal the plaster with a skin.  The gold sticks to the size,  the size sticks to the skin, and the skin sticks to the plaster.  If you put the size directly on the plaster, it just soaks into the plaster like a sponge and never gets sticky.

In my experiments I created skins using a variety of techniques.  I tried diluted acrylic paints, diluted oil paints, diluted oil paints mixed with a medium, and just plain Liquin.  The best results from my first batch of gilded coins was from the Liquin.

If you are not gilding and you just want something that looks good, the acrylic paints were great.  It is very easy to dilute them, and they dry very quickly.  That means you can put on several ultrathin layers in less than 30 minutes.  The layers are so thin that they do not affect the details.  The details become much more visible with the paint compared to white plaster.  I will include some pictures of painted coins when I make my next batch . . . .

I am a long time Calligrapher and for many years now have used Titanium White (Winsor & Newton Atists' Acrylic colour #349) for flat gilding (versus raised gilding on gesso). I use a thin mix and paint one coat over the area to be gilded -- it is much easier to use than egg-white glaire. The gold leaf adheres very well to the Titanium paint and can be burnished to a high lustre if desired.  I have used other makes besides Winsor & Newton with equally good results. You may still have to skin the plaster using Liquin before applying the acrylic paint.

James
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« Reply #17 on: May 18, 2009, 02:18:51 am »

I bought a set of uniface plaster casts of some mid to large bronzes some years ago. Busts etc. that I will never own. They had been painted to imitate various patination. Some of the paint has flaked in some areas but they are good enough to give you the idea.

Here is one of them (a large Probus, just under 43mm, and the reason that I bought the lot). The remainder can be seen in the following gallery.



http://www.forumancientcoins.com/gallery/thumbnails.php?album=1007

Regards,
Martin
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« Reply #18 on: May 18, 2009, 08:28:38 am »

I am long time Calligrapher and for many years now have used Titanium White (Winsor & Newton Atists' Acrylic colour #349) for flat gilding (versus raised gilding on gesso). I use a thin mix and paint one coat over the area to be gilded -- it is much easier to use than egg-white glaire. The gold leaf adheres very well to the Titanium paint and can be burnished to a high lustre if desired.  I have used other makes besides Winsor & Newton with equally good results. You may still have to skin the plaster using Liquin before applying the acrylic paint.

James


Not the best exemplar, but I used Titanium White to flat gild this versal "A" (not burnished).

James
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« Reply #19 on: May 24, 2009, 02:02:07 am »

I spent the day making a mess, but it was fun.  Before I started experimenting with the coins, I had only tried to gild something once before.  I need a lot of practice.  Here are the early results. 

A word about the results -- the final product looks much better life size.  These larger pictures show all the warts, all the flaws. 

What I learned:  The hardest part is making a good skin.  When making a skin, it's a game of tradeoffs.  The plaster is very porous and you need to get it sealed.  If you do not seal it, the gold size (which is the "glue" that you paint on and that makes the gold leaf stick to the surface) soaks into the plaster and the surface does not get sticky.  You can get a better seal and better adhesion with more coats, but more coats means sacrificing more detail.  I had the best results using medium yellow for the base coat to create the skin.  I tried titanium white but I lost too much detail.  As anyone who had painted before knows, there is a world of difference between the different colors.  Medium yellow is a good choice because it goes on evenly (even when watered way down) and dries fast.  After the yellow base coat I added one coat of Liquin.  Then I gilded it with patent gold leaf, which is much easier to work with than loose gold leaf.  The gold leaf is supposed to be 22k gold, but according to my acid tests, it is probably closer to 20k.  As you can see, there are a few tiny spots where the gold did not adhere.  I took that picture when it was overcast outside so the gold does not look very gold.  To help mask some of the flaws I decided to add some fake "patina" by holding it in a candle flame and getting some soot on the surface.  Then I rubbed off the soot and took the final picture.  The sun had come out so the color is much better in the last photo. 

I used the oil gilding method, so this cannot be burnished to shine and reflect.  I am too much of a beginner to try water gilding yet.  But I will keep practicing.  For now I think the best way to improve the final results is to try to make a better quality plaster cast.

My goal is to be able to make a display of my gilded casts so that I can enjoy my coins more -- even while the real ones are locked in the safe.^^ 


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« Reply #20 on: March 13, 2011, 09:44:01 am »

Here are my first results in plaster casting interesting coins for Curtis. Actually, this was the second attempt, since first time there were too many bubbles. Brushing the moulds with spirit did solve this problem but made the plaster adhere a lot more to the talcum-covered mould, so I had to destroy the moulds to get the casts out. All the rest worked very well, the casts on the left only look bad because the coin looks bad in the first place.

Rupert
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« Reply #21 on: March 14, 2011, 08:31:44 pm »

There is a commercial product to prevent bubbles in plaster casting called Airid.  I have not had any problem with sticking using it (but I have not made a cast for several years now so I really don't know if it has a shelf life).

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« Reply #22 on: March 14, 2011, 08:46:03 pm »

Casts sticking in the mold is a mystery to me: I've never had trouble removing the casts, whether or not I brushed in alcohol before pouring the plaster.

Bubble trouble should only occur with EF coins. Maybe the bubbles in your first batch resulted from too thick a mixture of plaster and water; use a little more water to make the mixture more fluid. And try to pour the mixture in slowly from one flat spot in the mold, so as not to entrap bubbles in the details.

I look forward to checking the obv. die on that unique Albinus As!
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« Reply #23 on: December 11, 2012, 01:14:35 pm »

Mr. Clay,

I just wanted to thank you for this post.  My kids and I have spent many hours making plaster casts using your method.  Not only have we done coins, but we have made casts of jewelry, beads, rocks, hand tools, ceramic flowers, nuts and bolts, football laces, shoe soles, dog/cat feet (the cat didn't like it so much), and we even made nameplates for the kids' doors!  It has saved us on several rainy and/or cold days when we couldn't play outside. 

Thanks again,
Cliff




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« Reply #24 on: December 11, 2012, 02:14:27 pm »

Cliff,

I am glad that you and your kids have found cast-making to be to be such a useful and even fun activity!

To give credit where credit is due: Michael Metcalf, a curator at the Ashmolean Museum, taught me the basics of casting, as I was about to set off on my first museum tour, spring 1967. A couple of years later, somebody else, I can't remember who, told me about the trick of pouring off most of the excess water before stirring in order to produce the right creamy mixture. Finally, around 1980, Hans Markus von Kaenel taught me how to use denatured alcohol to eliminate bubbles. I couldn't believe that that trick had remained hidden to me for so many years: I had earlier asked at least a dozen numismatists and museum curators how to eliminate the bubbles from casts of sharp coins!
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