|Monthly Tech-Tip |
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.
The bases of many artware pieces can actually be glazed and then fired on the kiln shelf without using stilts. How? A thin layer of silica sand and a super thin layer of clear glaze on the bottom - just thick enough for the melt to soak in a little and seal the body against water penetration (of course the side walls are the regular three coats). How thick? Just experiment. In this example, I watered down some Spectrum 700 clear and applied one quick coat. It does not seem like enough but it produces a glassy surface that picks up very few grains of silica sand during firing. In the worst case, when applied too thickly, some grains of sand will stick, but even then they can be rubbed off and the piece is still ok.
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 clay is Plainsman Snow fired at cone 04. August 2022 by Nina Berinstein. Very thin and light pieces can be made without danger of warping during firing (because this clay as zero fired shrinkage). This piece was treated with Mod Podge, this has sealed it against water penetration and not affected the appearance or texture.
Leaking of Fired Ceramics
ModPodge clay sealer
Sodium silicate on wikipedia
A sticky, viscous liquid. The most common deflocculant used in ceramics. Also used as a bonding agent.
Many ceramics are either porous by nature or by necessity. For example, stonewares need to be non-vitreous enough that they do not warp or blister on firing. Red earthenwares must be porous in order to have the red color (they go brown when fired higher). White talc or dolomite low-fire clay bodies always have high porosity. Bricks must have minimal firing shrinkage, which guarantees substantial porosity. Even porcelains can blister and it is common to cut back on feldspar to give them more margin for overfiring - that brings porosity. If water penetration must be prevented all of these need to be sealed, these are some of the methods.