A Professional Skills weekend was held at Aberystwyth Arts Centre on 18th/19th March as part of the ERDF funded Feile Clai project. 

The following are technical notes given out during the weekend by Jack Doherty who was leading the workshop.'

JACK DOHERTY     TECHNICAL NOTES

I use the Harry Fraser Porcelain body (1149) from Potclays The decorating clays are coloured with “home made” body stains.

These are mixtures of oxides modified with other materials which react in different ways with the soda firing.

For example, increasing the Alumina content of the recipe in relation to the Silica will produce softer, less shiny surfaces.

These colours are added to the dry clay body which is then reconstituted. Typical additions range from 1 to 7%.

I also make decorating clays which use the classic salt glaze slip combinations of China and Ball clays.

Orange/russet:     AT ball clay   33                           Yellow:  Porcelain 90

                            China clay     33                                       Titanium  10

                            Porcelain       33

Speckle:           Porcelain  98                                         Black:  Chrome  50

                         Illmemite   2                                                     Cobalt    20

                                                                                                Alumina Hydate     50  

                                                                                                Iron Oxide     60

                                                                                                Manganese Dioxide     30

Soft Green:   China Clay  40                                Dark Brown:    Chrome 25

                       Flint            35                                                  Iron Oxide 25                                    

                         Iron Oxide    8                                                  Manganese 10

                      Chrome       16                                                   Alumina  40

                      Cobalt           2

             Copper Carbonate    2                            

SLIPS

Orange Slip                                                                      Matt Slip                                                                     

Barium Carbonate  50                                                       AT Ball Clay 50

China Clay  50                                                                 China Clay 50

Base Slip recipe

China Clay 80

Feldspar 10

Flint 10

Bentonite 10

Add

a: 3% Rutile

b: 3% Cobalt

c: 0.3% Cobalt

     3% Rutile

d:  5%  Lilac stain

                                                            FIRING     

The work is once fired in a propane fuelled soda kiln. ( for design and construction details see “ Soda Glazing “ R. Tudball pub. A&C Black.)

I fire to cone 10 with a light reduction from 1000 C until the end of the firing.

The sodium solution is sprayed into 8 ports which are built into the front and back of the kiln.

I use a total of 2.5kgs. of Sodium Bicarbonate mixed with 9litres (2 kettles !)of boiling water.

The spraying begins just as cone 8 bends.

This amount of soda added over a two hour period gives a lightly glazed “flashed” surface which enhances the quality of the coloured clay surface.

The firing is completed with a soak of 15 minutes.