Found Moulds

 I want to try thrifted [charity shop] dishes to slump into. What should I know and look out for?

Material

• Ensure they are ceramic or stainless steel.

• Plastic and glass will not work. Differences in weight will help determine the material, as plastic is light and ceramic is heavy.

• Cookware, almost certainly, is stainless, but not all woks or metal serving ware are.

• If the potential mould is not one of these heat resistant materials, a cast can be taken to make a refractory mould.

Shape

• The items should have a shallow draft. 

• The sides of the proposed mould should not be vertical. The more gentle the slope the better. 

• A rim is better than none, as the glass can rest on a rim, but not so easily on an edge. 

Testing

• The items need to be tested for their resistance to the heat of forming to determine how they will perform.

• Take the proposed moulds slowly to about 50 degrees above the working temperature that they will be subject to.

Preparation

• A separator will be required to keep the glass from sticking to the mould.

• The surface of the mould has to be altered to accept the separator. This varies according to material.

• Ceramics most often must have the glaze removed, although occasionally unglazed forms can be found. At the least, the glaze must be roughened to allow the liquid kiln wash adhere. There is a formulation of emulsion paint and kiln wash that can be painted onto the glazed surface, and sometimes boron nitride can be used with out altering the surface texture. On some shapes it is possible to lay shelf paper on the glazed mould without further preparation before slumping.

• Steel moulds need to have the protective oils removed, which can be done at the test firing. Boron nitride (a high temperature lubricant) is the popular choice of separator by many. It is possible to use standard kiln wash as the separator by heating the form to a little above boiling temperature and spray or brush the kiln wash onto the hot metal. This may need repeated applications, although the kiln wash needs to be only a film. This heating and coating method can be used for glazed ceramics too. On nearly flat steel forms, it is possible to place shelf paper over the steel to form the separator as in ceramic moulds.

• The interior of ceramic bowls is not likely to be perfectly flat unless it has a broad base like a soup bowl. A disc of fibre paper that has a smaller diameter than the interior base can be placed so the glass will form a flat base, or with a smaller diameter disc, a foot.

Use

• Ceramic forms are most often used as slumping forms because the glass contracts more than ceramic. Draping over ceramic shapes can be difficult because of the different amounts of contraction on cooling can cause the ceramic mould to become trapped. It is possible to wrap steep sided moulds with 3mm fibre paper to provide a cushion between the glass and the ceramic.

• Steel forms are most often used for draping, because the steel contracts more than the glass on cooling. It is possible to slump in shallow forms or those with a shallow draft. It is also possible to place glass into a steep sided bowl below the height at which the sides become too steep. Careful levelling of of both mould and glass is required to be successful.

What are the Risks of Cast Iron Moulds?

There is a lot of concern about the safety of many products used in kilnforming, and much of it is based on hearsay. The best source for understanding the health and safety risks is Gregorie Rawls website.

Another, but more difficult to interpret, source is the SDS for each product.

Cast iron composition and safety

In this case the investigation is cast iron used as moulds. The first element is to know what cast iron is:

Cast iron is a class of iron-carbon alloys having a carbon content of more than 2% and silicon content around 1–3% with a melting point of 1,539°C (2,802°F). [Wikipedia]

The SDS gives the following information on Gray Cast Iron, the material of the cookware commonly used in the kiln:

• This material is rated as NOT HAZARDOUS by OSHA

• Appearance and Odor: Solid Mass, No Odor

• Specific Gravity: 7.86

• Boiling Point: 5000F

• 5 mg/m³ is the Time Weighted Average (TWA*) for fumes over an eight hour day. https://www.cdc.gov/niosh/idlh/1309371.html

These indicate there is no risk from fumes during casting firings as melting point will not be reached and the boiling point of cast iron is much higher than kilnforming kilns can reach.

The real risks are at room temperature, and are from the powder that may be created while grinding or smoothing the metal surface. The TWA* for cast iron dust is 10 mg/m³ over 8 hours. There are two alloy elements that also may be of concern – nickel and chromium. The amounts are low – chrome is from 0.5% to 2.5%, and even less nickel. The amounts are very low, giving little possible exposure.

The health concerns about using cast iron as a mould seems to be one of the misapprehensions of the amount of exposure, and therefore risk, that are common. The precautions are to have ventilation at source, use eye protection, and wear a N95 respirator.

The use of cast iron as a mould material

“Cast iron is a poor heat conductor compared to copper and aluminium, and this can result in uneven heating if a cast-iron pan is heated too quickly or… [unevenly].  Cast iron …[is] capable of storing more heat longer than... stainless steel pans. Slow heating... can lead to a more even temperature distribution. Due to the thermal mass of cast-iron utensils… they can retain heat for a long time...” Wikipedia.

This indicates that slower than usual ramp rates are advisable during the heat up to avoid breaking the cast iron through uneven heating.

Another thing to note is that the expansion rate [CoE] of between 106 and 114. The mould will contract more than glass, so preparing the mould with smooth sides and a sufficient draft is important to being able to remove the glass from the mould.

*Time Weighted Average (TWA) example:

"Rarely is exposure consistent throughout the day. Let’s say you are working in your studio for 8 hours grinding glass and exposure varies throughout the day… [Exposure varies in amounts]. The exposures throughout the day are averaged and the Time Weighted Average is determined. [In the example cited], … the OEL = 10 mg/m3 and the Time Weighted Average is 3.2 mg/m3, so actual exposure is below OEL (Occupational Exposure Limit)."    https://gregorieglass.com/general-information