|Monthly Tech-Tip |
The top two bars are the same stoneware fired to cone 6 (lower) and 7 (upper), having 4% and 2% porosity. The lower one is not vitrified, that is required for warm red (going higher turns it brown). Is the upper one more vitrified? No, it’s overfired and often bloats at this temperature. This clay begins melting aground 1% porosity.
The middle two bars are a terra cotta at cone 04 (lower) and 2 (upper). Is the top one vitrified? Although still having 2% porosity it is over-fired because it warps badly, bloats if fired only slightly higher and it’s LOI gases flood glazes with microbubbles and blisters. Is the bottom bar vitrified? No. And users don’t want it to vitrify, that’s what makes terracotta so stable and easy to use.
The bottom two bars are Polar Ice at cone 6 (lower) and 7 (upper). Both have zero porosity - strangely it fires whiter at cone 7 (at the cost of more warping without improvement in translucency). However cone 5 is actually better than both of these, translucency as almost as good, porosity is still zero and warping is minimal. In fact, if translucency is not needed cone 4 is even better (porosity is still well below 0.5%).
Adobe brick clay is simply the local clay that is used to make the adobe bricks. Almost always red burning but not always terracotta or earthenware. These clays are almost always suitable for pottery (with more processing). Here is an adobe brick clay from Sinaloa, Mexico. Locals have been making adobe bricks from this for 500 years. Adobe clays are generally just terra cotta bodies. But this one is not. These bars are fired from cone 2 to 7 (bottom to top) or 1125-1225C. They would be firing them to less than 800C, far lower. This clay actually qualifies as a stoneware but they use it as an earthenware. This is not as bad as it sounds. At 800C almost all clays are highly porous and pretty soft - that is actually an advantage for the method of construction used (e.g. watery mortar, easy cutting, good adherence of plaster, insulating value). These same bricks are used to build the kilns and they service well since temperatures never get high enough that they warp or crack.
A term used in the ceramics industry to signify the degree of vitrification in a fired clay. Mature clays are dense and strong, immature ones porous and weak.