Casting Notes for Silver Lost Wax Casting

This article “Casting Notes for Silver Lost Wax Casting” contains detailed notes of experiments done on silver lost wax casting.

Casting of silver can be fairly easy and these notes are the results of my experience through trial and error as well as instruction from an experienced jeweler and instructor.

Mark Ruby the instructor for the Fort Collins Senior Center was instrumental in helping with these notes and instructions.

The person that has helped me test and experiment with the different methods is Mary Jo Ziedler. We have made some mistakes and we have had our successes. Mary Jo and I have only performed investment and casting using centrifugal equipment.

My experience with vacuum casting has been with the help of Christen Gaylord. It was her equipment that was used in the experience with this method.

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All of methods of centrifugal or vacuum casting that I have performed have been based upon the lost wax concept. This is where one makes a model of wax, suspends the wax in a plaster like media called investment, fires the invested flask until the wax has burned out and then cast molten silver by centrifugal force or poured to fill the voids that were once wax.

The first instruction I have is to read all of the instructions, recommendations and warnings of all of the equipment and products that one will be using. Believe it or not this was one of the biggest helps I encountered in my experiences

Make a wax model

To start one need to have an exact copy of what one wants to be made of silver which usually is formed out of wax. There are many forms of casting wax that are available. There are sheet waxes that are great for thin flat pieces. There are waxes designed to be melted and drizzled over water for a very organic look. There are waxes that are used for injecting into molds.

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Organic items can be used in place of the wax as well. In our experimenting we have had some very unique items that were created out of twigs, pine cones, leaves and other organic items.

Sprue up the models

Once the models have been made they are mounted on a wax column that channels the silver to the models. This item is called a sprue. The sprue is generally a wax rod that attaches to a rubber base that holds the metal flask. The rubber base is called a sprue base.

When constructing the sprue tree, care and thought must be given to the positioning of the items on the tree for casting. They should be positioned so they are down stream from the sprue button and that no uphill flows are required. Think of this as if you are filling the flask up with water. Are there any parts that there may be a bubble form and the water cannot flow into it easily?

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Items that have a fine finish are best mounted with the fine finish side facing away from the sprue. At times small bubbles of air will form on the side facing down during the molding or investing process. The bubbles can mar the finish.

Weighing the sprue and the models

At this point one needs to weigh the completely assembled sprue and the models that have been attached to it. We need this weight later when calculating how much silver we need to melt in order to have a safe and accurate casting event.

Weigh the sprue assembly on a scale that can weigh in grams. Be sure to record this weight so it can easily be recalled later.
i.e. Flask #1 sprue assembly = 11 grams.

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Assemble the flask by installing the sprue into the sprue base and installing the metal cylinder into the sprue base. The flask is ready for investing.

Investing

This has nothing to do with monetary investments. Investment is the plaster like powder that is used to fill the flask. Once the flask is filled and allowed to dry it is placed in a kiln and slowly heated up to 1350 degrees F over about an eight to nine hour period.

Proper mixing of the investment is critical to the over all quality of the final product. Initially we experimented with various methods of mixing and then we read the instructions. Wow, what a difference it made reading the instructions. Who would guess the manufacturer would be the expert on their product?

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We use Kerr’s brand Satin Cast 20 investment and these instructions are for this product. (Note: these same instructions worked for the investment that is at the Senior Center.)

The manufacturer’s instructions are for a general amount and do not get very specific for the various sized flask. Using their measurements generally creates too much waste. Through experimentations and studying different books Mary Jo and I came up with a centrifugal casting ratio and measurement system that gets the investment mixture correct and with very little waste left over. I then took this system and through some experimentation came up the amounts for vacuum casting.

The following table has the levels of water that each flask should be filled to for the determination of the proper investment mixture.

This Table is for use with non-perforated flasks.

For Centrifugal Casting  For Vacuum Casting
Flask Height  % Water Level mm  % Water Level mm 
2 1/2″ / 63 mm 65% 41 mm 60% 38 mm
3″ / 76mm 62% 47 mm 58% 44 mm
3 1/2″ / 89 mm 60% 53 mm 55% 49 mm
4″ / 102 mm 60% 61 mm 55% 56 mm
5″ / 127 mm 60% 76 mm 55% 70 mm
If you are using the perforated flasks use the Centrifugal Casting column.
  • Place a collar of masking tape around the top of the flask. Typically the tape should extend above the top lip of the flask about 1 inch.
  • Fill the flask with water that is in the 70 F to 90 F degrees temperature range to the level determined using the water table above.
  • Once the flask is filled with water to the applicable height, pour the water into a graduated beaker that has 1 mille-liter increments.

The ratio for the investment mixture is 2.5 grams of investment for each mille-liter of water. i.e. The water volume was 320 mille-liters. Multiply 320 mille-liters times 2.5 which equals 800 grams of investment. (Take the mille-liters of measured water X 2.5 = grams of investment needed.)

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  • Don a dust mask or breathing apparatus to prevent the breathing of the investment dust. Weigh the investment to the proper amount.
  • Now you have the properly measured water and investment. Pour the water into the rubber mixing bowl. Next incorporate the investment very carefully trying not to create a lot of dust.
  • Mix the water and investment slurry for 3 minutes as per manufacturer’s instructions.
  • After the 3 minutes of mixing, vacuum the rubber mixing bowl and slurry place them in a vacuum chamber. Turn the vacuum chamber on. It should get very close to 25 inches of mercury vacuum. Once the slurry starts to boil time it for 20 seconds. At the end of the 20 seconds release the vacuum and remove the rubber mixing bowl from the vacuum chamber.
  • Carefully pour the pre-vacuumed slurry into the flask. The slurry should fill the flask to the proper level or at least very close to the proper level.
  • Place the flask into the vacuum chamber. Turn on the vacuum chamber and once the slurry starts to boil start timing. Vacuum the boiling slurry for 60 to 90 seconds. Typically I use 90 seconds.
  • Once the boiling slurry has vacuumed for the chosen amount of time, release the vacuum and turn off the vacuum chamber.
  • Place the prepared flask on a level surface and allow it to set up. Generally after about 15 minutes the flask’s partially solidified investment can be marked with a pencil or other item. The investment should be soft enough to allow it to be carved. I use the number of the flask that corresponds to my list from the previous steps. You did remember to make your list didn’t you?
  • Remove the tape from around the flask’s lip. When I cast with others I like to use my initials and numbers.
  • When the investment has solidified, 15 minutes to 30 minutes, carefully remove the rubber sprue base. It may take some effort.

I like to let my flasks dry over night as a minimum. At the Senior Center we typically let them dry for a week.

The Wax Burn Out

Using a kiln and the manufacturer’s recommended burn out schedule the wax is melted out and all the residues are vaporized.

The best kiln setup for this is a programmable kiln controller. With a programmable kiln controller it is pretty much turn on the kiln, start the program and wait approximately 8 1/2 to 9 hours before checking for the completion of the final hour at 900 Degrees F to start the melting of the silver for the casting process.

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If a non-programmable kiln controller is not available then one has to diligently attend the kiln and manually control the temperatures for the recommended amount of times. This can be a cumbersome process and tie one up for the whole burn out time period.

The burn out schedule for Kerr’s Satin 20 is as follows:
  1. 300 Degrees F for 2 hours
  2. 700 Degrees F for 2 hours
  3. 1350 Degrees F for 4 hours
  4. 900 Degrees F for 1 hour (Centrifugal casting it can be up to 1100 degrees F for 1 hour)
  5. Hold at 900 Degrees F or up to 1100 degrees F for centrifugal casting until all casting is complete (Note: Sometimes if one has some very fine details in the wax a hotter temperature up to 1100 degrees F can be used, but only when using the centrifugal casting process.)

The burn out should be complete and the flasks are ready for casting.

Work Sheet for Investing Flask For Lost Wax Casting

Wax weight = _____ Grams
Flax height = _____ mm
Water level needed = _____ mm
Water measurement = _____ ml
Investment needed = _____ Grams
Pour water into rubber mixing bowl: _____
Tape Flask: _____
Combine Water and Investment in rubber mixing bowl: _____
Mix investment slurry for 3 minutes: _____
Vacuum rubber mixing bowl for 20 seconds: _____
Pour investment into flask: _____
Vacuum flask for 60 to 90 seconds: _____
Let it setup and mark flask for identification purposes: _____

By Ken Moore - © 2009
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