Tuesday, June 21, 2022

Handbuilding at ORPG
Week 5: Surface Decoration 

Thursday, June 23, 2022

This is intended to show you a few basic strategies for treating your surfaces at the bone dry and leather hard stages. The pieces would then get bisqued and a more layers of colour will come in the form of glazes.

We will do four techniques on four small test plates:

Plate 1 (plain surface/no texture): Mishima
   - first layer: several random colour of underglaze, dilute a bit with water to make brushable
   - second layer cover with wax
   - third layer, after about 15 minutes and wax has dried draw out a design with your pin tool
   - fourth layer, apply black underglaze over design (work in well).
   - fifth step, with clean sponge wash away black, keep sponge clean at each sweep

Plate 2 (plain surface/no texture): Sgrafitto (means 'mark making')
   - look at Marcy Neiditz's work. Divide the plate into 2-3 areas.
   - cover the plate with 3-4 coats of underglaze colour (3 areas = 3 colours), allow for drying between
   - make a design with pencil (at a loss? think of Zentangle doodles). The inside areas should be filled
      with linear motifs like lines, circles, bean shapes etc. See pictures entitled sgrafitto samples.

Plate 3 (textured plate already prepared): Creating a Vintage look
   - apply one coat of black underglaze and allow to dry (it becomes matte).
   - With a clean sponge and a large bucket of clean water, start to wipe away the black. Keep the sponge
      clean and damp not wet. Aim to clear away the high points.

Background on Painting with Underglazes
Underglazes are slips (clay/water) made of clay and a colorant. Commercial underglazes might have the odd chemical addition for better brushability. 

This approach to colouring the surface is ancient. We see this with the Greeks, in Mesopotamia (Iran) and China. It is first seen in geometric linear designs and evolved into larger colour areas. The colorants were minerals and the basic colour palette included the following:
Red - red ochre
Yellow - yellow ochre
Black - manganese
Blue - cobalt
Green - Copper

Contemporary Underglazes
At the time of the Minoans and Greeks coloured slips were composed of fine particles of clay with water. The recipe for this is found with articles on Terra Sigillata. The finest particles of clay were used, a colourant was added and finally the piece was buffed. This approach was found in Minoan and Greek works.

Today additional ingredients are added to the basic mix. In part because we fire hotter and our clays are more complex than terra cotta. Here is a typical Underglaze recipe:
Janet DeBoos White slip
Custer Feldspar 25%
Silica 25%
EPKaolin 24%
OM4 Ball Clay 24%
Bentonite 2%

For Blue:
Custer Feldspar 25%
Silica 25%
EPKaolin 24%
OM4 Ball Clay 24%
Bentonite 2%
Cobalt Carbonate 2.5%

For Black
Custer Feldspar 25%
Silica 25%
EPKaolin 24%
OM4 Ball Clay 24%
Bentonite 2%
add Mason Stain Black 6600  8%

Experiment with:
Copper Carbonate for  Green at 2.5%
Chrome for green at 1.5%
Red Iron Oxide at 3-4%

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