Tennessee Technological University - Appalachian Center for Craft - Clay Studio

Vince Pitelka, 2007

All-Temperature Decorating Slip

  There are abundant recipes for general-purpose decorating slips, and a popular notion stipulates that any white porcelain claybody recipe thinned to a slip will work well as a base slip for decorating.  The caveat is that such a slip will often work well, depending on the particular application and the colorants added.  The decorating slip recipes we use are designed to accommodate different temperatures and firing conditions and varying additions of colorant, and thus give a higher incidence of success.  These decorating slips are designed primarily for midrange and high-fire work, but will also work reasonably well at low-fire.  A popular high-fire porcelain recipe contains equal parts kaolin, ball clay, potash feldspar, and flint.  I have modified that recipe as indicated below to make it work better as a decorating slip. 

  There is a fundamental problem with any all-purpose base slip recipe, arising whenever making black and brown slips, which might contain 10% iron oxide.  Red iron oxide is refractory (melt-resistant), but any reduction firing and all high firing will convert it to black iron oxide, a very powerful flux.  A slip base formulated from a standard porcelain body might contain 25% potash feldspar, and the addition of 10% iron oxide will produce a serious flux overload, often resulting in boiling, blistering, and crawling at high temperatures.  To address this problem, we use two versions of the base slip recipe.  One contains 20% potash feldspar, and is intended for all colors except black and brown.  The second version, intended for black and brown slips, contains 12% potash feldspar in order to compensate for the addition of the iron oxide. 

  Both slip recipes contain 5% 3134 frit, a high-boron sodium-borate flux.  Occasionally, slips will lift from the surface of a piece during bisque or glaze firing, especially when applied thickly as in trailing, feather-combing, or layering, and/or if the surface of the clay beneath is very smooth.  Boron is a powerful flux that becomes active during early red heat, and in this case helps with early adhesion, reducing the chance of the slip lifting from the surface.  It also improve general performance in low-fired work.  Many base slip recipes include 5% borax to supply the boron, but borax is a soluble alkaline flux, and thus will deflocculate the slip.  3134 frit is insoluble. 

Flocculation and Deflocculation
Some people prefer to use deflocculated decorating slips, which perform well for thin applications such as polychrome slip painting.  It is true that deflocculated slips contain less water and thus experience less drying shrinkage, but the addition of a deflocculant causes like ionic charges in the particles in suspension, and thus they repel one another, making the slip flow much more readily.  That is no advantage in a decorating slip.  Our slip recipes are flocculated by the addition of one half of one percent Epsom salts.  Addition of a flocculant causes varying ionic charges in the particles in suspension, making them attract one another, which encourages the slip to stay in place during and immediately after application, and discourages adjacent liquid slips from spontaneously intermixing upon application. 

  To assure opacity even in fairly thin applications, we add 8% Zircopax (or any other zircon opacifier).

All-Temperature Decorating Slip
Base Recipe for White and All Colors except Black and Brown
EPK - - - - - - - - 34
OM-4 - - - - - - - 20
G-200 - - - - - - - 20
3134 Frit - - - - -  5
Flint - - - - - - - - 21
Total - - - - - - - 100
Plus
Zircopax - - - - - -  8%
Epsom Salts - - - 0.5% (dissolve in hot water and add to the recipe before adding cold water to achieve desired consistency)

Base Recipe for Black and Brown Slips
EPK - - - - - - - -  34
OM-4 - - - - - - -  20
Calcined EPK - - - 8
G-200 - - - - - - - 12
3134 Frit - - - - - - 5
Flint - - - - - - - -  21
Total - - - - - - - 100
Plus
Zircopax - - - - - -  8%
Epsom Salts - - - 0.5% (dissolve in hot water and add to the recipe before adding cold water to achieve desired consistency)

Slip Colors from Ceramic Oxides
Tan/Gray - - - - - - - - - - 5% Rutile
Light Green - - - - - - - -  1% Chrome Oxide
Medium Green - - - - - -  4% Chrome Oxide
Dark Green - - - - - - - - - 7% Chrome Oxide
Light Blue-Green - - - - - 1% Chrome Oxide, 1/2% Cobalt Carbonate
Dark Blue-Green - - - - - 4% Chrome Oxide, 3% Cobalt Carbonate
Sky-Blue - - - - - - - - - -  1/2% Cobalt Carbonate
Medium Blue - - - - - - - - 2% Cobalt Carbonate
Dark Blue - - - - - - - - - - 5% Cobalt Carbonate
Light Brown - - - - - - - -  3% Red Iron Oxide
Medium Brown - - - - - -  6% Red Iron Oxide
Dark Brown - - - - - - - -  9% Red Iron Oxide
Black - - - - - - - - - - - - - 8% Red Iron Oxide, 5% Manganese Dioxide, 5% Cobalt Carbonate
*Saturated Iron - - - - - - 25% Red Iron Oxide (thin applications only – see below)

*The saturated iron slip is specifically intended for thin applications on greenware where you want the iron color to come through the glaze.  A good example is on white shinos, where this iron slip will come through and create a red-brown or brown color.  Also, when you want to achieve brush-painted iron brown marks or banding on any glaze, you are often better off doing the brushwork with the saturated iron slip on greenware rather than with an iron oxide patina over the raw glaze coating.  When painted underglaze on bisqueware, brushed iron oxide can cause serious glaze crawling.  Overglaze, the fresh iron marks are so easily smeared before firing.  Using the saturated iron slip on greenware eliminates both problems.