Casting an Historic Architectural Detail




Introduction: Casting an Historic Architectural Detail

The architectural details for this crest were cast and replicated using a silicone casting compound and water putty. The crest is an 85-year old historic detail located at the stately Bently Reserve in San Francisco's financial district.

Step 1:

This was the broken crest needing replacement.

Step 2:

This was an intact shield.

Step 3:

First, a method needed to be devised to cast a mold around an existing, unbroken, shield crest. To facilitate this, a contour guage was used to measure the wall details around an intact shield. These measurements were then used to create a form box to contain a silicone casting compound which would encase an unbroken shield.

Step 4:

The form was then fastened to the wall using two #10 wood screws and clear silicone. The silicone was used as a gasket to seal the form, and lessen leakage when the RTV casting compound was poured in. As evidenced in this image, a bit of seepage was still inevitable.

Step 5:

After curing for forty-eight hours, and being removed with a screw driver and a sharp knife, the result was a mold which was highly detailed and relatively heavy; weighing about fifteen pounds, but was still somewhat pliable. Due to the casting material's weight, the form must be mounted securely to the wall.

Step 6:

A simple and inexpensive "water putty" was used to cast the final product. The powder and water were mixed to a pancake batter consistency and poured into the level mold.

Step 7:

Hardening time for this product will vary depending on the thickness of the casting. We waited 24 hours, although the product specifies only eight hours for hardening. As an additional precaution, to prevent breakage, a piece of inexpensive nylon screen material was cast inside of the shield.

Step 8:

After the cast had dried, the edges were cleaned and excess material was removed. Unlike wood or porous clay, the water putty will not stain. However, it can be painted.

Step 9: Finished Product

The final product epoxied in place.



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    13 Discussions

    Excellent instruction for designers.

    This is a great! I'm restoring and old building now and could put this to good use!!

    A 15 lb silicone mold??? It says Be nice down below.. so..
    Such molds are not poured, they are painted on, with thick (thixotropic) formulations of the urethanes and silicones. A light and quick mothermold is slapped on.. my mold might use 1-2 lbs. No fancy pouring box.. but I must hail your dogged good process.
    You have made a giant production mold when only one casting was needed.
    I would have cast the part in Hydrocal + glass fiber, takes maybe an hour. Any plaster would be fine when the ornament is out of reach.
    Finish: gloss is harder to match than color!

    Yes, I forgot to include an illustration of the contour gauge and its application. Getting that box profile contour was probably one of the trickiest parts of this process!!

    Great work; nice job on the molding box. The only quibble I have (and I am sure it is in part due to the picture), is that the painted cast piece looks nothing like wood and a lot like plastic in the final image. The cast could have been placed into a 3D scanner and then routed using a CNC machine - or more affordably, using a CarveWright or CompuCarve unit (both are the same thing, CompuCarve is Sear's name for reselling it with minor cosmetic changes) - there is a 3D scanner attachment in it which can read in the details of the cast piece, and then the unit could reproduce it in wood... that way it could even be made of the same type of wood and stained to match.

    1 reply

    The original shield was not wood, but a clay or terra cotta type material. The paint used was a bit too glossy, but looks fine from down below (high ceiling!). I like your high tech solution, but that wasn't in my budget!

    Can this mold be used more than once. where do you get the casting compound how big can a mold be made?

    1 reply

    Yes, the mold can be reused. It is a solid, durable (and heavy!) rubber mold now. I think we got the casting compound from McMaster-Carr; it was a ten pound bucket.

    Excellent instructable. Would you describe contour gauge and how it works to make box? Have you used other silicone materials (a cheap alternative I've used for casting is silicone bathtub caulk!--does RTV Casting Compound smell as terrible?) Lastly, why water putty instead of plaster--is it similar?

    1 reply

    The casting compound did not stink like bathtub caulk fortunately, but it was very expensive. The water putty seems to be stronger than plaster, and easier to work with at denser consistencies too.

    Question about step 4: the photo shows the box tightly sealed against both the wall and ceiling- so how do you get the silicone casting compound into the box? I'm guessing that there is a hole you inject it into, but it seems that this would be the most difficult step- please clarify/describe this part in better detail.

    1 reply

    There was some space between the box and the ceiling... barely enough to pour it in with a little cup.