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Christine Homer Weaving

2006
Workbook

Wood Firing,  Salt Glazing
& Pit Firing

All  workshop glazes will be ready
when you arrive. The recipes provided below are for your
 reference and records

The principle for attending this workshop should be to increase your knowledge of different firing methods.

I suggest reading
 “Towards A Standard”  the first chapter of  Bernard Leach’s A Potters Book.

Preparing for the Workshop  

To determine cubic inches multiply H x W x D

A bowl  5" tall ,  6" wide, and 6" deep is
5" x 6" x 6" = 180 cubic inches

One cubic foot = 1728 cubic inches

2 1/3 cubic feet =  4000 cubic inches

2 chambers x  4000 cubic inches =
8000 cubic inches

Clay Suggestions

  Amherst White Stoneware
Salt &  Wood Fring
Light Color

B-Mix
 &
B-Mix with Helmar Kaolin

Suggested Shapes
of
Pots

 This wadding prevents pots from fusing to the shelves, which  happens in soda atmospheres created by wood ash, and salt.

Wadding also allows the flames to travel under pots for additional flashing.

 Bowls made with thick rims and same diameter can be stacked rim to rim.

Pots stacked and fired in this manner are often  enhanced by flashed areas around the wadding.

Think about the kind of pots you will make for the wood and salt kiln.

  Look at pictures in books and choose forms that use these firing processes to their fullest advantage.
 

Alumina Hydrate 50

Cornwall Stone         46
Whiting                     34
EPK                         20
Tin Oxide                  4
Copper Carb             4
               

Pit Firing

  Participants are allotted  "Two Square Feet" of space in the Pit firing,
about 18" x 18".

The Clay Body

When making pots for pit firing, finish throwing with metal rib to
smooth the surface of the pot.

Pots for the pit firing need to be bisque fired and burnished.
 The "Pit Firing"  begins with bedding pots in sawdust at the bottom of the kiln. Copper Sulfate is added and firewood stacked to a depth of about three feet.

          The firing takes about 16 hours. When removed from the kiln the pots are cleaned and a wax coat applied to protect the finish and enhance the color and shine.

Normal stoneware bodies usually do well in pit firing. Unlike Raku clays, which are often coarse, it is best to use a fine grained body in Pit Firing to insure a polished surface.

            Colors produced in the firing are reflected against the clay color of your pot.  A White Terra Sigillata gives the most dramatic results.  A Peach Terra Sigillata offers a more subtle contrast. 

Bisque Fire Your Pots Prior to Arrival

     Bisque the pots to be pit fired to cone 010-08.
Higher bisques (over 06 ) can dull the shine.

Terra Sigillata

Terra Sigillata

            Terra Sigillata is a slip applied to the surface to a pot, which fills in small pores in the surface with a light buffing.

 A smooth finished pot with a highly burnished surface will sparkle with color when Pit Fired

            Potters who specialize in the technique may spend a great amount of time preparing Terra Sigillata.  The process can be complex, however the one  Robert uses is quite straight forward and simple.

          White Terra Sigillata, begins with a white clay of a fine particle size, do not use fire clays or grog.

             Edgar Plastic Kaolin is the coarsest clay in the following receipt, it is used for its whiteness. The Ball Clay has finer clay particles, and Bentonite  the finest sizes of all.  Calgon serves as the deflocculant. 

The deflocculant  encourages coarse particles to settle and can then be removed from the slip. 

Peach

E.P. Kaolin       1170 grams
Alberta  slip        270 grams
Bentonite              60 grams
Calgon                   8 grams
Water                    14 cups

 White             

E.P. Kaolin       1170 grams
Ball Clay            270 grams
Bentonite              60 grams
Calgon                   8 grams
Water                    14 cups

Mixing

Applying

     Weigh and mix dry ingredients together, then add to the water/calgon solution.  Stir well  and let stand overnight. 
 The slip will settle into layers,  the top is thin and watery, the middle smooth, and the bottom has the most coarse particles. 
This will not be obvious to the eye.

            The trick to getting a good Terra Sigillata is to take out the middle third of the container while not disturbing the top and bottom layers. 
     Using a Turkey baster, squeeze the air out of the bulb, insert half way down in the slip, and release the bulb to draw in the Terra Sigillata. 
Press the bulb “before” putting the tip in, so you do not blow air into the solution and thus stir up the layers.

       Repeat the process until you have removed the middle third of the slip.  This is the Terra Sigillata you will use to burnish your pots.  Discard the rest.

 Terra Sigillata should be applied to a smooth bone dry pot.

           Using a Haku ( or any wide soft bristled ) brush apply the Terra Sigillata to a small area, about 5 square inches.  Apply in a way so you feather  the slip at the edges. Do not let drips run over the un-slipped body.  Before the Terra Sigillata drys, while still dark but NOT wet, rub with a clean soft cloth until it shines.

             There is no need for hard rubbing provided you catch the Terra Sigillata at the right stage. Two coats  provide the pot with a smooth even surface.

 Potential Problems

    If the Terra Sigillata is too thick, or if it is applied in too many layers, it will craze  when drying or in the firing. In extreme cases the Terra Sigillata will flake off the pot.  This effect can often be seen in older Native American pottery.