|Monthly Tech-Tip |
On the loss of talc earlier this year we had to reformulate a low fire white burning body to use dolomite instead, recipe L4410P (like talc it raises thermal expansion to ensure fit of commercial glazes). As its advantages and disadvantages become evident we have been documenting them on the Snow page. A recent revelation has been the matter of rehydration of the limestone (dolomite is ground limestone): Bisque firing dehydrates it. The dolomite particles are neutralized somewhat by being isolated and having reacted to some extent with neighbouring clay and feldspar particles. Further, during dehydration, they leave considerable porosity into which they should be able to reexpand later if needed. This photo demonstrates something we have not seen in our dry climate: These 3D-printed bisque pieces have spontaneously cracked after sitting around for some time in the much damper climate of southern Ontario. In some cases, swelling occurs around the cracks. Until we can further tune the recipe to chemically tie up the dolomite take some precautions when using this type of body. Glaze ware soon after bisquing. Dry it as quickly as possible after glazing. If any surface has not been glazed then render it impervious to water penetration by using a silicone sealer. Photo courtesy of Nilou Ghaemi, Sheridan College.
The right side of this bisque-fired clay bar (which has about 15% porosity) has been surface-treated with a silicone sealant. It repels the water, the drop rolls around on the surface if I tilt the bar. The drop on the left side absorbed into the clay in seconds.
This body has high porosity, almost 25%. It is L4410P, a dolomite-based low-fire whiteware, Plainsman Clays makes this as a product named "Snow". But this high porosity has some advantages, one of them is that it soaks up silicone sealer well. The slip-cast piece on the left was sealed (you can see the surface sheen) and it is impermeable to water penetration (the glaze is not crazed so water cannot penetrate there either). The piece on the right soaks up water readily (on the lower unglazed portion). Sealing this specific body is doubly important because the dolomite particles within can rehydrate over time, especially in damp climates, causing pieces to crack. Even the foot rings of functional pieces should be sealed, not just to prevent hydration but also waterlogging.
An inexpensive source of MgO and CaO for ceramic glazes, also a highly refractory material when fired in the absence of reactant fluxes.