Vulcan Black Stoneware (Medium) 1200-1260C

Vulcan™ Black Stoneware (Medium) 164-VUM | Recommended firing 1200-1260C / Cone 5-7

Recipe Adjustment: From batch number 2322 (week 23, 2022) onwards, recipe adjustments were made to the Vulcan recipe due to unavailability of a raw material. SIO-2's primary advice is to use glazes with a medium-high thermal expansion (further information in the technical notes below). Through our own tests here at Potclays, we recommend not to exceed Cone 6, and if your controller will allow, it is beneficial to programme a controlled cooling segment <50c/hr from 200-50C.

Vulcan Black Stoneware is unlike any other on the market, providing an unusual intense brown-black colour when fired in an oxidising atmosphere. In a reducing atmosphere a metallic finish with shades of blue are obtained. Containing a medium grog (up to 35 mesh) Vulcan Black Stoneware Medium has been formulated especially for artistic ceramics. It behaves excellently during drying/firing and its high plasticity makes it ideal for modelling. It is widely used for murals and sculpture and is ideal for handbuilding. Supplied in a 12.5kg bag.

Typical Data:

Firing range: 1200-1260C / Cone 5-7
Biscuit: Cone 06 (1855ºF)
Texture 4-5 on our indicative texture scale, smooth1 coarse 10.
Water content: 21%
Plasticity (IP Atterberg): 22
Carbonate content (CaCO3): 0%
Drying shrinkage: 7.0%
Firing shrinkage at Cone 7: 4.5%
Porosity (water absorption) at Cone 7: 9.4%
Dry bending strength: 2.0 N/mm2
Fired bending strength at Cone 7: 27.5 N/mm2
Thermal coefficient (a25-500ºC): 55.8x10-7ºC-1

Technical Notes:

Vulcan Black Stoneware (Medium) is not recommended for tableware due to the grog content. Please see Vulcan Black Stoneware (Fine) instead.
Vulcan clay bodies are non-toxic according to European Rules (CLP) and also American Rules (ASTM D 4236). The manganese dioxide content in Vulcan bodies is lower than 5%: Under the CLP regulation, the manganese dioxide content required for classification as toxic must be greater than 25%.
Use zinc-free (Zn-free) glazes to prevent unexpected reactions such as bubbling or colour changes. Other elements that can react are zirconium (Zr) and tin (Sn). We have found that Mayco zinc-free clear glaze is suitable for use with Vulcan bodies and does not seem to suffer from the usual problems caused by outgassing from the high oxide levels in black bodies. Many ^6 commercial glazes such as Mayco perform well on Vulcan but trial-and-error is needed. Also see our potclaysglazes Instagram feed/highlights.
SIO-2 warn against using glazes with a low coefficient of thermal expansion (a25-300ºC <60x10-7ºC-1). The Vulcan clay fired at high temperature has a high coefficient of thermal expansion in the range of 100 to 200ºC, which causes tensions between the glaze and the body, mainly in pieces glazed on one side only, which can cause breakage during cooling in the kiln or even later (for example, when pouring hot water inside). Check with your usual glaze supplier to ensure that you use glazes with a medium-high coefficient of thermal expansion (a25-300ºC >60x10-7ºC-1).
Vulcan Black Stoneware is no longer recommended for use in casting slips due to the high plasticity which makes it difficult to correctly deflocculate.