|Monthly Tech-Tip |
Traditional kiln patching or bisque fixing products made by mixing refractory grog and sodium silicate can have amazingly low drying and firing shrinkages: We measure ~0.5% from wet to dry and ~1% from dry to cone 6 for a 30:70 sodium silicate: 48 mesh kyanite blend (because the kyanite has a particle size distribution that enables dense packing and it expands when fired). Here I tried filling the large gap in a bisque mug handle that was cracked completely in two. The kyanite stirs easily into the sodium silicate, it wets all particle surfaces rapidly. While the mix is not plastic it does have plenty of cohesion and can be formed and pressed into recesses. It hardens on surfaces (even your hands) quickly. Notice that the non-porous nature of the kyanite gives poor glaze coverage here, I could have alleviated this by applying a thin layer of the clay over the repair (with CMC gum and possibly a little frit for fire bonding to the kyanite below). Given the density at which this compound fires at cone 6 no other additions seem necessary. One user even adds white glue to non-fritted versions of this to aid in adhesion.
Kyanite is a granular material used in the manufacture of ceramics and abrasives. It is notable for low thermal expansion and one-way expansion on heating.
Potters and some manufacturers fire ceramic ware twice, once to prepare it for glazing (call bisquit firing) and the second time to melt the glaze onto it.