Nitride-bonded silicon carbide kiln shelves are much thinner (5/16"/ 8 mm) and lighter than regular cordierite/mullite shelves. Not only are they are easier to handle but their lower mass saves on heating costs. SiC Shelves stay flatter for longer, even with heavy loads. This is the recommended size for 10-sided top-loading kilns like Euclid's R-153 or R-200.
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Manufacturer's (IPS Ceramics) guide for safe nitride-bonded kiln shelf handling and optimum kiln shelf performance:
Dry before firing, a typical drying cycle being 5 hours at 90°C (194°F). Kiln furniture products can absorb water during manufacture, transportation, or storage. Wet items may crack if exposed to temperatures of more than 100ºC (210ºF) as the absorbed water will create uneven rates of heating across the product.
Handle with care. Thin silicon carbide shelves can be more brittle than traditional cordierite shelves so must be handled carefully. Any bumps, bangs or other impacts during handling the shelves (or on loading and unloading the ware) must be avoided as these can often cause a shelf to instantly break.
Storage. Shelves should ideally be stored on edge. If stacking flat, limit the number of shelves to prevent cracking from the cumulative weight. Be aware of the risk of moisture absorption during storage. Storage areas should be dry and it is good practice to lift any shelves slightly off a concrete floor using thin wooden strips (to avoid chipping the edge and to minimize the risk of water pick-up from damp concrete or spills.
Correct size of shelf. IPS Ceramics recommends that shelves are sized correctly so that there is a 50mm (2”) gap between the edge of the shelf and the kiln walls (to ensure proper heat distribution in the kiln). Shelves should not be placed directly on the kiln floor but should be raised off the floor by at least 25mm (1”) using props / spacers.
Thermal cracking. Silicon carbide shelves are not as resistant to thermal cracking as traditional cordierite shelves. We would recommend reviewing your firing cycle when changing to silicon carbide. As a guide, heating / cooling rates up to 5ºC per minute can be used, however larger shelves (or if you are placing large, heavy ware) may need a slower firing cycle. Particular attention should be paid to potential points of rapid cooling (e.g. opening damper vents, starting cooling fans, ‘cracking’ the kiln door).
Glaze firings. A thin coating of an alumina-based wash is recommended for glaze firings to prevent glaze drips from strongly bonding with the surface of the shelf (items must be dried after coating). Heavy glaze contamination can be removed in most cases by using a grinding wheel or by sandblasting (safety equipment must be used to avoid breathing the dust created). Silicon carbide shelves are not recommended for salt glaze use, as the shelves can develop a thick ‘foam’ coating.
ELECTRICAL SAFETY - Silicon carbide shelves can conduct electricity; ensure shelves do not come into contact with any part of the heating elements in electric kilns to prevent short circuits and the risk of electrical shock.