|Monthly Tech-Tip |
The term Slip can have various meanings in traditional ceramics.
In ceramics, this term can refer to a number of things:
-A clay slurry poured into molds to be cast into shapes. The slip is deflocculated to minimize water content and fine tune viscosity. The deflocculation process involves using special chemicals that enable you to create a fluid clay-water slurry with a very low water content.
-A mix of clay and other minerals and fluxes that is applied to dry or leather hard ware (e.g. pottery, tile, brick) to produce an enhanced surface, either to increase quality, reduce permeability by water, improve glaze coverage, add a design motif or improve fired hardness. The slurry is often flocculated to gel it and improve its ability to hang on to the surface during drying. Often referred to as an engobe.
-A simple mix of water and clay that is used as a glue to attach leather hard or dry elements together (e.g. Handles to mugs, spouts to teapots).
This is part of a project to fit a fritted vitreous engobe (slip) onto a terra cotta at cone 02 (it fires harder there). Left: On drying the red body curls the bi-clay strip toward itself, but on firing it goes the other way! Right: Test bars of the white slip and red body compare their drying and firing shrinkages. Center back: A mug with the white slip and a transparent overglaze. Notice the slip is going translucent under the glaze. Why? It is too vitreous. That explains how it can curl the bi-clay bars toward itself (it has a higher fired shrinkage). So rather than add zircon to opacify the slip, it is better to reduce its frit content (thereby reducing its firing shrinkage). Reducing the frit in the slip will also make it more opaque (because it will melt less). Front: A different, more vitreous red body (having a frit addition) fits the slip better (the strips dry and fire straight).
These are fired bars of Barnard Slip going from cone 04 (bottom) to cone 6 (top). It is melting at cone 6. Porosity is under 3% and the fired shrinkage above 15% from cone 1 upward. Drying shrinkage is 4% at 25% water (it is very non-plastic). The darkness of the fired color suggests higher MnO than our published chemistry shows.
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).
L3685U slurry was applied to the insides of both of these mugs. But on the left it is a "slip", on the right an "engobe". Why? The left mug only has a thin layer, applied by painting a gummed version on (at leather hard stage). On the right a gelled slurry was poured into the leather hard piece, poured out and the rim dipped (creating a much thicker layer with more power to impose its own drying and firing shrinkage). So it is much more important that the latter be compatible with the underlying body. The EBCT test is used to measure how compatible the body and engobe are.
In ceramics, glazes are suspensions. They consist of water and undissolved powders kept in suspension by clay particles. You have much more control over the properties than you might think.
A method of forming ceramics where a deflocculated (low water content) slurry is poured into absorbent plaster molds, forming a layer against mold walls, then poured out.
A technique used by potters to decorate ware. Neutral and colored slips, clay in thick paste form, are trailed onto the ware (in raised lines and strokes) to create tactile designs.
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.
Glaze Types, Formulation and Application in the Tile Industry
An technical overview of various glaze type used in the tile industry along with consideration of the materials, processes and firing.