|Monthly Tech-Tip |
L3685X white slip (left mug) has 5% more frit than Y (right). The frit is a melter, creating more glass bonds to adhere it to the body (it also hardens it and darkens the color a little). But the frit also increases firing shrinkage, 'stretching' the white layer on the body as the kiln cools (the slightly curled bi-clay bar demonstrates that). However the glaze, G2931G, is under some compression (to prevent crazing), it is therefore 'pushing back' on the white slip. This creates a state of equilibrium. The Y slip on the right is outside the equilibrium, it flakes off at the rim because the bond is not good enough. Adding more frit, the other side of the balance, would put the slip under excessive tension, reducing ware strength and increasing failure on exposure to thermal shock (the very curled bi-clay bar in the front, not this clay/slip demonstrates the tension a poorly fitted slip could impose).
This is part of a project to fit an engobe (slip) onto a terra cotta at cone 02 using the EBCT test.
Left: On drying the red body curls the bi-clay strip toward itself, but on firing it goes the other way!
Right: SHAB test bars of the white slip and red body enable comparing their drying and firing shrinkages.
Center back: A mug with the white engobe and a transparent overglaze. The slip is going translucent under the glaze because it is too vitreous. Its higher fired shrinkage curls the bi-clay bars toward itself. Reducing the frit will reduce the firing shrinkage and make it more opaque (because it will melt less).
Front: A different, more vitreous red body (Zero3 stoneware) fits the slip better (the strips dry and fire straight).
The term Slip can have various meanings in traditional ceramics.
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.
Creating a Non-Glaze Ceramic Slip or Engobe
It can be difficult to find an engobe that is drying and firing compatible with your body. It is better to understand, formulate and tune your own slip to your own body, glaze and process.