Freeing the Casting
During this step of the breakout process, it’s necessary to shatter the investment to free the casting. There are basically two methods to shattering the investment: Wet and dry.
- Wet Breakout: Obviously, the difference between these two methods is the water. The use of water rapidly cools the casting and aids the removal of the investment. Many jewelers opt for this method because it’s fast and easy. Here are some techniques for wet breakouts:
Technique #1: Briefly submerge the hot flask in a container of water, lift it out and repeat the process until the casting has cooled to near the temperature of the water.
Caution: This technique can cause hot investment to fly out of the containment area and hit the caster. It's recommended this be done in an enclosed area.
Technique #2: For a more rapid cooling of the casting, dunk and leave the flask in water and wait until it cools near the temperature of the water.
- Dry Breakout: Dry breakout can be done either hot or cold.
No matter which method you choose, it’s important to be aware of the period of time when the grain boundaries of the cast metal are relatively weak due to the elevated temperatures. One thing to know about the metal as it cools is that everything is not solid at the same time. The higher melting point metals solidify first, followed by those with lower melting points. If the metal is subjected to vibration stress at this critical time it can break apart along these weak boundaries. This condition is often termed hot tearing.
If the metal is allowed to cool to below 600°C (1100°F) before beginning the breakout the likelihood of experiencing hot tearing is lessened significantly. This is dependent on a large extent to the specific alloy formulation, and the melting point of the lowest eutectic formed in that alloy. This condition is most often seen in the nickel-based white alloys, which are broken out hot.
Proper Investment Handling and Disposal
Another potential health risk is that the steam rising from the quenched flask can contain silica that may be inhaled. It is good practice to wear the proper protective equipment when breaking out castings. Most casters are good about wearing respirators when mixing investment but many feel that since the investment has bonded after casting there is little health risks, a mistaken supposition.
When it comes to investment disposal, dry investment is considered to be inert, and can be disposed of as a solid waste in most public landfills.
Wet investments, however, are considered to be liquid or hazardous waste, and need to be disposed of in a specialized waste center. There are some circumstances when a wet investment can be allowed to dry through evaporation, so it becomes dry instead of liquid waste. NOTE: Under no circumstance should the wet investment be dumped or flushed down a drain, or poured out back behind the shop. Large casting houses and manufacturers generate liquid waste in quantities that do not allow for evaporation to be practical so they have to be more vigilant in the creation of solid or liquid waste.
Though investment disposal is similar in most States, it’s still best to consult your local regulations for the proper disposal of both wet and dry forms.
Next up in this article series on casting breakout: Part 3 – The Reveal. It focuses on flasks and the finished casting.