|Monthly Tech-Tip |
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.
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 is a common sealer available at a hardware store. I have dipped the terra cotta tile and it has dried. The surface of the dipped portion is smoother. It also has a slight sheen and better red color. This sealer even makes it possible to use a porous clay body for outdoors, terra cotta bricks have long be protected against freeze-thaw spalling using products like this.
Available at Walmart, Amazon, Michaels and others. It is water-based and non-toxic. There are many kinds, one is "dishwasher safe" and another is for outdoor use. It dries to a hard finish. This makes it potentially useful to seal porous ceramic such as terra cotta and dolomite or talc white-burning bodies, even functional ware.
The product claims to be an economical, easy-to-apply, penetrating sealer that resists most common oil-based and water-based stains. Unlike other sealers, it will not leave a difficult-to-remove residue on the surface. Low-odor, water-based formula. Water sheds off ceramic surfaces and this has worked on even highly porous clays having up to 25% porosity.
Sodium silicate can be diluted in water and will have some sealing properties when absorbed into porous clay surfaces. But, of course, it will shrink as it dries, thus not completely sealing. However the product "Liquid Quartz" is claimed to be very effective for ceramics because it is doped with nano-sized particles of quartz. It is claimed to be a non-toxic, food-safe, water-based sealer. It is also much more expensive than other options.
Although Duncan Ceramics is under new ownership its products are still available online. They have multiple sealer products and many years of experience.
This mug is made from a test clay body firing to about 3% porosity. While plenty strong it is capable of absorbing some water. But a thin layer of transparent glaze on the base solves that issue. I applied it by adding water to a brushing glaze version of G2926B transparent and carefully painting it on. A thin layer of silica sand on the kiln shelf completely solved the problem of sticking. And there are no sand grains stuck on the bottom of the piece either.
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.
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.
A type of red firing pottery. Terra cotta clay is available almost everywhere, it is fired at low temperatures. But quality is deceptively difficult to achieve.