March 30, 2023 - Glazes and Glazing

Last week we introduced glazes so that this week we could finish our work. This discussion will continue below (II).

I. Underglazes and "Low fire firings" (Temperature range cone 06, 05, 04)
In the last few weeks, we used Underglazes because of the wide range of colours available as well as, decorative techniques like sgrafitto and mishima. Our initial coloured test plates and tiles were fired (bisque fire:cone 05); touch ups were made and they were refired with a clear glossy glaze (1 coat of Duncan Pure Brilliance) and fired again (cone 05 - here we are not actually doing a "bisque" since the clay has already been vitrified but we are keeping the same low fire temperature because our glaze Duncan Pure Brilliance is made for low fire as per direction on the bottle. Many potters choose to do all of their work in low fire because of the colour brilliance  and Greener Environmental concerns.

II. Midfire Glazing (Temperature range cone 5, 6)
At the guild we do midfire cone 6 glazing. We discussed how glazes are essentially fluid glass once heated to the right temperature. A glaze is composed of three basic parts: glass, a flux (controls melt) and stabilizer (clay: controls drip).

These glazes are applied mainly by dipping into the bucket or pouring. Generally the rule is no thicker than a dime. The glaze must be well stirred before use and check with your finger that you see you nail as demonstrated in class,

       Above: ORPG Glazes - 2-3 greens, 2 blues, matte purple, tan, brown-red

What is a Glaze?

Glaze: glass + stabiliser + melter Glass: silica/flint/quartz Stabilizer: clay (alumina) Melter: Flux in feldspars

Glass formers

SiO2 - Silica - our main glass former

Al2O3 - Alumina - a very high temp glass former and glaze stiffener

B2O3 - Boron - low temp glass former (Gerstley Borate; ferro frits: 3124; 3134; 3110; 3195)

Clays (stabilizer) – keeps the glaze in suspension
● EPK (Edgar Plastic Kaolin) Al2O3 + SiO2
● Grolleg kaolin (china clay)
● OM4 Ball clay
● Red Art clay
● Bentonite - a powerful suspending agent

Primary fluxes (R2O) - get the melting started
    Primary Fluxes                                                                          Sources

Li2O - Lithium Na2O - Sodium K2O - Potassium ‘

Lithium Carbonate Nepheline Syenite Custer Feldspar Spodumene Cornwall stone Soda Ash

Secondary Fluxes (RO) - keep the melting going     Sources (RO)

MgO - Magnesium CaO - Calcium SrO - Strontium BaO - Barium ZnO - Zinc

● Whiting (calcium carbonate) CaO ● Wollastonite (calcium silicate) CaO, SiO2 ● Talc (magnesium silicate) MgO, SiO2 ● Dolomite CaO, MgO ● Magnesium carbonate MgO ● Zinc Oxide ZnO ● Strontium carbonate SrO ● Barium carbonate BaO ● Bone ash (tri-calcium phosphate) CaO, P2O

 

March 16, 2023 - Surface Decoration- Sgrafitti and Mishima

 This week, we consider a few decorative strategies using slips and underglazes. These strategies are used on green ware (or unfired clay).

Artists Shown in Class

Nancy Meiditz - her sgrafitto lines are bold and not likely done by a pin tool but a specialized carving tool (see Diamondcore for example).

                   Above: Janet DeBoos (sgrafitto on black underglaze and on terra sigilatta. Also uses decals                     which are applied after the glaze firing at 015- 018).

Above: Susan Feagin layers on many techniques including sgrafitto and mishima, but also newsprint transfers and more. Look her up on UTube.

Above: Adero Willard layers many areas of texture and colour. Foe example the light blue background with squares and arabesques are done using the mishima technique. Other areas are painted and wax resisted. Her strategies are discussed in Pottery Illustrated.

Techniques shown in Class Demos:

Class technique 1: Sgrafitto
The geometric patterns are drawn onto black underglaze (3 - 4 coats). If the clay is white, the incised lines are white. Note the great balance of black areas and the various widths of lines (several engraving tools).

Class Technique 2: Mishima 

This is done by scratching a design into the wax. When the wax dries, a black underglaze is applied into the lines. Allow it to dry and scrape or sponge off. Do not let the wax dry more than 30 minutes otherwise the lines donèt get through the wax.

In this picture (above) the wax has been applied & the design is drawn with an Exacto. I find it easier to do my initial drawing in pencil on the bisque, then cover with wax - this allows me to see the drawing as I cut through.

Above: The black underglaze was applied to the drawing. Each of three coats was allowed to dry before the next application. A sponge is used to clean up the surface but a scraper can also be used (the latter is the traditional Korean approach).

Various coloured underglazes are applied to the shapes.

About Slips.... 

Because we are still working with greenware objects, we are decorating with underglazes and slips which are structurally similar to one another.

Slips are applied leather-hard or bone dry greenware. However, the slip used on bisque usually follows an engobe recipe (see below). 

The example used in class for pure slip decoration was a cup by David Miller (UK). He often uses a white base slip and then adds further layers of coloured slips. I remember him saying once - "what you see is what you get.." referring to the mason stains added to a slip. (Try it out: place 3 tbs of white slip in a small container. Choose a mason stain colour and start with 1/2 a teaspoon on your lid, spritz and mix then add to the slip. Continue doing this til you reach your desired colour.

Above: David Miller Slip-based decoration

Comparison of a Basic Slip Recipe - Any Similarities?

Susan Feagin - White Base Slip  (Cone 04-9) Custer Feldspar  27 EP Kaolin 34 OM4 Ball Clay 20 Silica  19 -------- Zircopax  5

Sunshine Cobb Base Slip Cone 04-9 Kaolin  34 Ball Clay  20 Potash Feldspar  27 Flint-silica  19 Zircopax  8 Soda ash  0.25 Sodium Silicate  0.25 ____________________

Robin Hopper: White Base Slip Cone 04-12 in Ceramic Spectrum): Ball clay 75 Kaolin 10 Silica 10 Feldspar 5

Colorants are added to the slips. There are two categories:
1) Minerals/oxides 2) Mason Stains.

 I. Common minerals/oxides:  Rutile (tan); Cobalt carbonate (blue – 1-3%); Iron Oxide (red-up to 15%); Copper Carbonate (green – up to 10%); chrome (darker green 1-3%). These are available at the guild.

 2. Mason stains: infinite - These are available at the guild.

Slip-Engobe for a bisqued surface

Replace base slip with Engobes on Bisque Ware

Engobe recipe R Hopper :

Raw Material

Cone 04-3

Cone 4-6

Cone 6-10

Kaolin	20	15	15
Calcined Kaolin	10	20	35
Talc	25	10	5
Calcium Borate (or Frit 3110)	15	10	0
Nepheline Syenite	0	10	15
Silica	15	20	15
Borax	5	5	5
Zircopax	10	10	10
Total

 
Popular Request: Crack Mending (paper clay slip)

3 Tbs powdered clay (let scraps dry out and crush in a plastic bag)
6 squares of toilet paper (pull apart to 1 ply) torn into small bits
2-3 Tbs Water
1 drop of Darvan 7

Directions
Place drop of Darvan 7 in 2 Tbs of Water
Break up toilet paper and let disintegrate  in water
Mix powdered clay a bit at a time til disired consistency for filling in cracks

 

March 11, 2023

This week we return to our Darted piece and consider our options for surface decoration. We will continue making our texture stamps. We will add decorative handles and sprigg molds using our magic slip (darvan 7 in the mix) to our darted piece. Next we will look at how to apply Underglaze paint and Underglaze slips. Linda will do a demo on how to make an Underglaze slip following this recipe:

Slip for Underglaze Painting 04-9
Susan Feagin

Custer Feldspar  27 EPKaolin     34 OM4 Clay    20 Silica    19 ------ Add Zircopax  5 Colorants Tan: Mason stain 6129  6% Rutile 2% Yellow Green: Copper Carb  5% Mason stain Yellow 6404 Class Demo: 2 Mason Stains  are chosen by class

                                             

                                           Artist: Susan Feagin who currently teaches at Penland

Strategies for Underglaze Painting

  Nancy Meiditz  (above)
Adero Willard (below)