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Mold making for exact replicas


#1

I have a question to pose for all. I have been hand-fabricating
silver for some time, but have just decided to take the plunge and
add some cast pieces/elements to my repetoire. Here is my question:

I have two pieces of 1000 year old bronze pieces of jewelry that I’d
like to make exact replicas to wear. They are both in relatively
good condition, missing only a portion of the pin on the back on one
and an attachment ring on the other. I know I’ll have to recreate
this part of the pendant missing the pin, so basically I’m dealing
with a flat backed piece. The other is hollow (but with no back).
Adding a attachment is no problem.

I’d like to make a mold, make a wax of it and then cast them in both
bronze and in silver.

What’s the best approach? I’m concerned about several things:

  1. how do I attach a sprue that will withstand the mold making
    process (rtv or vulcanized) but not damage the piece? (I don’t want
    to solder a rod to the pieces!_

  2. what can I safely put on the pieces as a release that won’t
    damage the patina of the originals?

  3. I’m concerned about nicking the original when cutting the mold
    due to my inexperience. Would you recommend see-thru moldmaking
    materials? If so, what product?

As always, I appreciate any advice/help.

Keep up the good work!!
Chris Hanson
Ketchikan, AK


#2

Hello Chris;

Concerning your problem of molding those 1000 year old pieces.
Here’s how I would do it, but you should wait until some others have
answered and get other opinions to weigh mine against.

email Michael Knight of Castaldo for help with their products.
Locate him at http://www.castaldo.com

Castaldo makes a room temperature 2 part silicon rubber compound
that sets in about 15 minutes. It has the consistency of putty or
stiff clay, and it doesn’t seem to stick to surfaces unless they are
coarsely porous. I’d fill half a mold frame with an amount made up
of the 2 parts that has been kneaded together. Before it sets, push
the article into it level with the surface, and do likewise with the
rod and sprue former that you will use. Get the rod as close to the
article as possible. Take a bic pen top and push in four
indentations in the four corners of this half filled mold about 1/4
inch deep, or place 4 of those brass locator buttons. These will be
the mold locks to keep the two halves aligned. When it is cured,
leave the original in place and dust it with a release agent such as
talc or corn starch. Pack the mold with more of the mixed 2 part
rubber to the top of the mold frame and clamp metal plates on either
side of the mold frame to pressurize the rubber in the mold slightly.
When it is fully cured, you should be able to simply pull the two
parts of the mold apart without having to do any cutting. If there
is a little rubber between the sprue forming rod and the cavity
formed by the original, simply trim it away with an exacto knife.

Help others make informed buying decisions with Castaldo . We
welcome your opinions and experiences with ordering, customer
service and and over all satisfaction.

Write an Anonymous Review
http://www.ganoksin.com/resources/review.php?id=1799

David L. Huffman


#3

Chris,

Since these piece are 1000 years old and most likely very
collectable do not use any of the rubber that needs to be heated to
form the mold. This will alter the color of the Bronze. There are
several brands of RTV rubber that can be used.

As for the sprue, if you use one of the RTV rubbers that can cure at
room temperature, you can use glue to attach the sprue and then
release it with Acetone or heat the sprue rod to release it.

If you work slowly with your knife, making short cuts until you
reach the piece, you will reduce your chance of a mistake.

Good Luck
Greg DeMark
email: greg@demarkjewelry.com
Website: www.demarkjewelry.com


#4
    I have two pieces of 1000 year old bronze pieces of jewelry
that I'd like to make exact replicas to wear. 

Greetings Chris,

First, you didn’t mention the thickness nor area of the pieces, this
could make a difference in the molding technique, Generaly speaking,
RTV will give less shrinkage than a vulcanized mold, Castaldo makea
a Silicone called No-Shrink Pink that if donbe under extreemly close
temperature tolerance will give a near exact size wax pattern. they
also make a very good RTV putty that gives fair results regarding
shrinkage.

Zero-D makes a very good RTV in a Liquid that has as far as I can
tell NO noticeable shrinkage, and Bill is a nice guy to deal with
and an Orchid member, as is Mr. Knight from Castaldo.

I use Zero-D’s RTV to mold my CadCam waxes, and have had excelent
results, I have about 40 molds that I have done for one customer a
lot of the molds have had six to eight hundred waxes injected and
the detail is still good.

As far as attachiung a sprue, if you go with a liquid RTV then you
can cold fuse (sounds more professional than glue) the sprues to the
pieces,

The Patina should not be affected by the RTV it would in the
Castaldo Pink under vulcanization, and you would have to solder your
sprues.

Hope I’ve helped a bit,

Usual disclaimer, Not affiliated with either company just satisfied
customer of both

Kenneth Ferrell
www.shadras.com


#5

Dear Chris,

I used to do a lot of period reproductions for the Minnesota
Historical Society. A lot of old Hudson Bay items the trappers used
to trade for furs and such. Handle with white glove etc. Call me and
I’ll tell you how I do it. Fridays are the best. 651-227-3921.

Best Regards,
Todd Hawkinson


#6

Chris, I use Akron Silicone RTV whenever I want to copy a piece. It
has Zero shrink, is clear, and works beautifully every time. Good
Luck, Craig