Making a Mold for a cold cast bronze

I just recently completed my Alto Relief sculpture  ‘ Moonlight Sonata’ 30″x 20″ and have decided to mold it for a cold cast bronze. Here is the steps, pictures and reference information I used for this process. I researched on the smooth- on website for similar sculptures to determine from the wide range of products they sell and decided on the best product for my Alto relief. I am going to use brush on 40 as I do not have a vacuum pump and many of the products they sell require one during the process.  The molding products you decide to use is determined by the size, material of your original (is it clay, concret wood etc) and what medium your final copy will be cast ( casting resin, concrete, metal, cold cast bronze?).

Reference:  Has technical information – excellent videos and step by step processes. Also a tech line available to answer questions. I found the advice and support EXCELLENT!
smooth- on technical support phone # (800)762-0744 or at (610)252-5800 or contact them directly on their website and use a digital form for questions.

continue reading or watch the youtube video Here:

Step by step ( using a different product list but the process is the same)
Materials list:

Brushes (disposable one use)
containers (graduated for easy measurement)
papertowels (cleanup)
Spray on shellac or super seal from smooth-on
Universal mold release
Smooth-on Brush on 40
Box of disposable rubber gloves
disposable stir sticks ( I used paint sticks but advised to use metal)
Thickener Cab-o-sil (mix with brush on 40 for undercuts 3rd layer)
So strong pigment ( color pigment every other layer to determine proper coverage of layers)
Safety Mask:

Step one:
Complete a final review of your sculpture for any necessary changes – once you start you cannot stop until the process is completed. According to the technical product information once the layers have cured completely you cannot add further layers as adhesion will be difficult. You will be adding layers onto ‘tacky’ rubber layers with a 45 minute interval between each layer.
Prepare the Model:
Ensure it is secure onto the surface  (glue with a glue gun)
(my model was leather hard clay stuck to a plastic sheet)
Spray the model with spray shellac

Let this coating of sealant (used to cover any porous areas)  dry thoroughly and ensure you have covered all the undercuts and small spaces.
Once the model is dry to the touch (about 30minutes +)  Spray on a light mist of universal mold release. Lightly cover the entire sculpture with a brush to ensure the mold release has covered the entire sculpture. Otherwise you run the risk of the rubber not lifting clean off of the model once the mold has been finished.
Let this dry for about 5 minutes and then mist another layer of the universal mold release over the entire model.
Let this completely dry for about 15-20 minutes.

Step 2:
Mix thoroughly equal parts A and Part B of the brush on 40 to cover your mold. I was surprised that the amount I actually required was much less than I expected. Consequently I mixed and used more than I needed -I put on a heavy layer and had excessive air bubbles. I had to remove this layer and then do a thinner layer repeating the process. IT IS VERY IMPORTANT TO DO A THIN LAYER FOR THE FIRST LAYER OF RUBBER.

 [  IF you notice in the above picture there is a TIMER – IMPORTANT remember you only have a working time of 20minutes to apply the layers – thin is better than thick – apply and mix in smaller batches if necessary or  more than one person applying the layers helps as well.  My husband and I worked on this together to get the first layer applied within the time frame.  ]

LET THE FIRST LAYER SET UP (GEL ) for 45 minutes PRIOR to next application of the brush on rubber. Test the rubber by touching lightly with rubber gloves the mold should be tacky and release from your finger – if not it is NOT ready for another application.
Third step:
Apply another layer of brush on 40 (mixing equal parts A and B) add pigment [this will assist you in ensuring complete coverage of layers)

IF you are doing a relief you will want to add ‘Keys ‘ to the mold by spraying a ice cube tray with the mold release and filling with the brush on 40 mixture and letting it set up. Release from the tray and place after the third layer around the mold.
Step four: Undercuts and adding keys –
Mix equal parts A & B and add cab- O- sill till the mixture is like a bread dough
Apply to undercuts and under the mold keys from the ice cube trays.


After first layer:

After keys have been added and undercut thickener:

Picture below –After last 2 layers of Brush on 40 (NO thickener added to last 2 layers)BETWEEN LAYERS LET SET FOR 45 MINUTES – MUST BE TACKY (TEST)BEFORE ADDING ANOTHER LAYER .

LET THIS CURE (SET /DRY) FULLY FOR 16HRS BEFORE MAKING THE MOTHER MOLD. The rubber mold should be approximately 3/8″ thick.
Making the rigid support or ‘mother mold’:

I have completed to this point and my model is in the curing stage the following is excerpted from the smooth- on website temporarily and will be replaced with my own personal pictures when I complete the following steps:

Using a sharp razor knife, cut excess silicone from the outer edge of the mold. Create straight edges and square off all corners.

Spray the rubber mold and surrounding surface with Ease Release® 200 to separate the support shell from the mold. In some cases, Smooth-On’s Sonite® Wax may be necessary to release surrounding surfaces from the support shell.

Creating the rigid support shell: Smooth-On’s Plasti-Paste® is a trowelable urethane resin designed to be used for support shells. It is mixed 1 Part A (liquid) to 2 Parts B (paste).

Combine 1A:2B and mix thoroughly.

Apply Plasti-Paste® over cured Mold Max® Stroke®.

Cover the entire mold with Plasti-Paste®. Important: Plasti-Paste® has a working time of 8-10 minutes. Do not mix more material than can be applied in this amount of time.

Mix and apply more Plasti-Paste® as needed. Be sure to apply material beyond the edge of the silicone mold.

Apply at least 3/8” (1 cm) thickness, making sure rubber mold is thoroughly covered. Large molds may require added thickness for support shell stability.

Wooden blocks are attached to the support shell with additional Plasti-Paste®. These blocks will help level the mold during casting and allow for easier handling.

Once all Plasti-Paste® has fully cured (approximately 90 minutes), carefully remove the support shell from wall.

Edges of the support shell can be sharp. Sand Plasti-Paste® to make handling safer and easier.

Seat the rubber mold into the support shell. The mold keys added in the previous step will make aligning the mold with the shell very easy.

Making a cold cast bronze video:

Alto Relief – Moonlight Sonata

Here is my recent work & my attempt at Alto Relief – Sonata 30″ x 20″ sculpted with a porcelain clay – still in progress. Current photos and more updates to come. I will be cold cast bronzing this piece so will show the process for cold cast bronzing. Most people do not realize that Alto Relief is a rare form of sculpture and one of the most difficult art forms to master and to my knowledge it is not taught in any of the fine art schools. Low relief is also more difficult followed by 3d , painting and drawing as you do not have to alter the axis for any of the elements. I believe Low relief is taught in the last year of most fine art schools after students have learned painting, drawing, and 3d. I advise every artist I know to work in all of the art forms from low relief – high relief – painting – drawing and 3d. Each builds on the strengths of the others. It is a synergistic relationship. Artists that work in all the mediums in the past became great and I believe it is because they did all of the art forms rather than just focusing in one area – Take Michealangelo for example he sculpted and painted throughout his life and I am thoroughly convinced that this was the key to his success. So I plod on ….. My Alto Relief…

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