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Tutorial - How to Cast Pewter in Silicone RTV Molds


Back at the World Con in Chicago in 1982, I picked up an Imperial disk in the dealers room.  Now, some 19 years later, I am building some new costumes and needed some new disks.  Since I have done a lot of pewter casting, I choose to cast new pieces in pewter ( using a Silicone RTV mold ) rather than turn new ones on a lathe.  The pictures below document the process I used to make the new Imperial Officer disks in pewter.  I also have a listing at the bottom of the page for the suppliers I use.  

IMPORTANT SAFETY NOTE:  

I DO NOT USE LEAD based pewter alloys, NEITHER SHOULD YOU!  The toxic effects of lead in pewter alloys (especially on the young) are well documented.  Don't take a chance with your health.

While lead based alloys of pewter have been used historically, there is no reason to use lead based alloys in this day and age.  I currently I use an pewter alloy called OR-8, it is a tin, antimony, and copper based mix.  OR-8 has great strength and flows well into detail. As a side note, OR-8 is also very close to the pewter alloy also known as " Britannia Pewter ".  There are a number of "non-lead" pewter alloys available, so experiment and find the one that works best for you.  Also, keep a notebook of your pewter casting projects.  It helps you to keep track of what did and did not work well while casting and what you did to solve it.

Pewter Casting and Silicone RTV Mold Making Equipment  

Safety First -  Pictured are some of the "Tools of the Trade" used when casting pewter .  I always wear at least a dust mask.  While the pewter alloy I use does not "fume", the dross particles (the scum you get a the top of the pot) can become airborne and get inhaled.  Though small and non toxic, you really don't want them in your lungs.  Gloves and safety glasses protect your hands and eyes from spills of molten pewter (if and when they happen).  Be safe and you won't be sorry.  I also use a vent hood to help pull any pewter dross / particles out of the work area.

 

Silicone RTV and catalyst for creating the Imperial Officer " Disk" mold.  These can be bought at a mold supply or special effects supply store.  
A gram scale to measure components.  You can buy them for about $30 at cooking supply stores.  This one was bought at Williams and Sonoma (spelling).  It is accurate to 1 gram.
My smallest (2 lb) Lee melting pot, ladle, and lead thermometer.  These items can be bought at most gun supply stores.

A shot of the lead thermometer I use to track the temperature of the metal.  The RTV silicone will take over 600 degrees.  However, it will degrade quickly at higher temperatures (it turns to a Jell-O like consistency).  The thermometer costs around $30, but it is worth its weight in gold.

 

The Pewter Casting Project ( Imperial Officer Disk )

Pictures of the Original - Here are some images of the original Imperial Officer disk that I bought in 1982.  It looks like it was turned in aluminum and is drilled to accept a small machine screw.

Making a Silicone RTV Mold ( 2 part mold )

I used a two part silicone RTV mold compound to create the gravity molds I used for the project.  While spun molds can be used, I am not currently set up to do them. Mold Number 1 (the rightmost mold) was an unsuccessful two-part mold.  It poured vertically from the top edge of the medallion.  The metal "chilled" too much in pouring and the edges/ridges nearest the top would not cast nice and sharp.  

Example Pewter Mold, Silicone RTV Mold and Pewter Castings

Creating the Mold with non-sulfur clay.  The RTV will not cure correctly if you use a clay with sulfur in it. Creating Pewter Mold Side 1
The finished mold box for Mold 1.  Make sure to seal all edges or your silicone will quickly show you where your leaks are. Finished Mold Box for Side One of the Imperial Disk Pewter Casting Mold - Side 1
A shot of the mixed RTV.  The scale is "zeroed" with the cup in place.  The Silicone is then poured in and then the proper amount of catalyst is added.  (The knife is not left in when checking weights for the RTV and catalyst). Weighing the Silicone RTV for the Pewter Casting Mold
I first brush in a layer of RTV directly against the model.  This helps to eliminate air bubbles in the surface of the mold. I then pour in the rest from a corner (slowly to avoid trapping air) Pouring the Silicone RTV into the Mold Box for Side 1 of the Imperial Disk Mold
After the first layer cures (about 8 hours), remove the first layer of clay, add mold release (VERY IMPORTANT), rebuild the box and pour in Silicone RTV to cast the second side.  If you don't use mold release, the Silicone RTV will fuse with the first side, making it very difficult to get a clean mold line (you can cut it by hand, but what a pain).

 

Preparing to make side two of the Silicone RTV mold box for Pewter Casting

After the second layer cures (about 8 hours), you can strip away the clay, carefully open the mold, and clean it.  You are then ready to blacken/dust the mold and start casting.  Unfortunately, Mold 1 just would not cast the upper most ridges of the disks.  While I did work with Mold 1 to try to fix it (increased the size and the position of the spru, raised the casting temp, changed the alloy used), it just would not fill the upper edge of the casting correctly.   

 

Picture of both of the Silicone RTV mold sides

Making a Silicone RTV Mold ( 3 part mold )

I then created a three-part mold (the Mold  on the left) that would cast the piece face down (helping the metal to fill all the edges while still very fluid).  It worked much better and the castings from it come out nice and sharp.

 

Example of the 3 part Silicone RTV mold for casting pewter Imperial Officer disks

The mold box for mold number 2, I used Plexiglas side walls to show a different way to build the mold box.  Again, makes sure to seal all of your edges or the silicone will drain from the mold box.

 

Side one of the pewter casting mold box

Brush in the first layer of RTV with a brush.  I use the brush to make sure that the layer of RTV next to the model fills in all cracks and crevices of the model.  This also helps avoid trapping air bubbles next to the surface of the model.

 

Brushing in the Silicone RTV into the mold box

The finished three-part mold.  I molded the top half in one piece and then created the spru.  I then cut the top part in half (to allow the spru to release).

Finished 3 part Silicone RTV mold for casting pewter Imperial officer disks

I use graphite to dust/blacken all of my RTV molds for pewter.  To blacken the mold, spray in a bit of graphite powder and brush it in to crevices of the mold.  Then rub it in with your fingers.  I normally blacken the mold every five castings.  The dusting/blackening of the mold helps you get an almost mirror finish right out of the mold.  If you don't dust the mold you get weird little pits/bubbles on the surface of the mold.

 

Blackening the silicone RTV mold to cast pewter

I sealed/closed the mold using rubber bands.  I have also used wooden frames and clamps on other molds, but the rubber bands work well on this one.  Be careful if using rubber bands, if molten metal gets on them, they will disintegrate and release part of the mold.  Always wear your safety gear and cast inside of a secondary or tertiary containment tray/box.

 

Molten pewter, pewter casting pot and sealed mold

Voila, a cast pewter Imperial Officer disk with spru intact!  If you cut of the spru (the excess material) while it is still warm, the pewter cuts more easily.

 

Cast pewter Imperial Officer disk with spru

Here is one of the cast pewter pieces with the spru cut off.  Notice that the finish is very bright without even buffing.  It will get brighter once it is buffed.  Also, I made the second mold with the center hole filled in the master.  This was to help the upper disk mold properly.  The center recess is then drilled into the piece.

 

Cast pewter Imperial Officer disk with no spru

Some of the finished cast pewter pieces.  The original Imperial Officer disk is in the lower left corner along with its machine screw.  Next is the back of a finished pewter disk with a butterfly clasp.  The two pewter disks on the bottom right have machine screws inserted.  The top two rows are finished disks.  I will be taking the best 3-5 disks from the top group and use them to make what I call a "gang mold".  The gang mold will cast 3-5 pewter pieces at a time rather than one at a time.  In addition, the pieces will be cast with the center hole, rather than drilling the center hole in each piece.

 

Bunches of cast pewter Imperial officer disks

A before and after shot showing the front and back of a cast pewter Imperial Officer disk before final sanding/drilling and the finished disk with the center recess drilled (from left to right).

Before and after of cast pewter Imperial Officer disks

Another shot of some finished pieces.  The left piece has the butterfly clasp installed.  The butterfly clasp is normally used to mount the disk on hats or others soft items.  The alternate method of mounting is to use a machine screw (i.e. for a belt buckle).  I am working on ways to cast the screw as part of the disk, but for now I will drill them and mount them using the machine screw (just like the one I already have)

More finished pewter pieces
   

Links to Pewter Casting Suppliers

Pewter (OR-8 alloy)

Conquest Industries

9915 Bell Ranch Drive
Santa Fe Springs, CA 90670
Phone: (562) 906-1111
Fax: (562) 906-1112
Toll-free: (800) 368-CAST
Email: sales@conquestind.com <mailto:sales@conquestind.com>

http://www.conquestind.com/default.htm

 

Silicone RTV

CASTINGS, A DIV. OF REB TOYS, INC.
P.O. BOX 298
Eastsound WA 98245-0298
(360)376-3266 (8:30am-5:00pm pacific coast time.)

For orders ONLY call:
1-800-346-0567
Fax: (360) 376-3280
http://www.miniaturemolds.com/mainframe.htm