Ceramic Slip
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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).
How to test if an engobe fits a clay body
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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).
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It is a clay, a very non-plastic one. These are fired SHAB test bars of Barnard Slip going from cone 04 (bottom) to cone 6 (top, where it is melting). Porosity is under 3% and the fired shrinkage above 15% from cone 1 upward (second from bottom). 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 (and also higher iron). The white areas on the lower temperature bars are soluble salts.
Since this is a fine particled material, it could likely be made plastic with a bentonite addition, likely 5% or more would be needed. Solubles could be precipitated using barium carbonate.
Creating a Body/slip Equilibrium in Terra Cotta Ware
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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).
The difference between a slip and an engobe
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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.