Copper Matt Technique:
The bisque-fired pots are "glazed" using a reactive copper matt slip which gives an iridescent range of colours. They are fired in a propane gas-fuelled kiln to just under 1050 degrees c. Once the slip begins to fuse they are removed from the kiln with tongs, placed on a circular bed of sawdust for a few minutes and then sealed with a metal bin. This creates a post-reduction effect, and I repeat it two or three times at intervals and the copper colours begin to change and work their magic.
I like to encourage a blackened area to develop around the base of the pots and grounds them. This creates a kind of landscape horizon. The dramatic copper lustre colours from the reduction, therefore develop into the appearance of a rather surreal and "other worldly" sky. This technique works hand in hand with the slab-built forms, as each facet of the pot provides a blank canvas for the colours to develop.
Copper matt slip is one of the hardest Raku techniques to master. (If not the hardest!) Much is dependent on the elements, the right blend of sawdust, the correct kiln temperature, the right size reduction bin, the length of reduction/re-oxidisation and even the weather! (Cold in my experience is best!) No two pots will ever be the same, and it is virtually impossible to achieve the same range of colours twice. Yet- this is all part of its' enigmatic appeal and ensures everyone who owns one of my pots has a completely unique item. The copper matt slip recipe that I use is a variation of John Wheeldon's recipe.
John Wheeldon slip Recipe
(Spray or pour)
100 copper oxide (or double copper carbonate)
10 High alkaline frit
1 Polycel wallpaper adhesive ( stops slip rubbing off at bisque stage)
Raku Glaze Method & Semi Volcanic Raku crater
I also use a copper alkaline recipe which is sprayed or applied thickly to the pot. The work is then heavily reduced in a metal bin packed full of wood shavings and smoked for up to an hour to create rich cobalt blue, copper, pewter and metallic textured surfaces. Interesting effects can also be obtained by flashing the glazed pot in a reduction bin lined with newspaper or magazine print for up to half an hour.
1 Geoff's Raku glaze recipe:
High alkaline frit 65
St borax frit 25
Tin or zirconium silicate 20
Copper oxide (between 2 and 5 percent- double if using copper carbonate)
Recently I have been developing a crater glaze technique for raku. It is perhaps at best a semi-crater effect- but can be higher fired at around 1030c which develops a harder pin-holed type effect- or pulled at a lower temperature, around 980- 1000c which develops a more bubbly surface. This can be carefully abraided using a stone to reveal a cratered look. These pots can be buried in wood shavings for reduction or flash reduced in a sealed tin with newspaper or magazine sheets. This technique can occasionally develop gold from the copper present in the glaze. Although it is quite rare! This effect is a work in progress!
2 Shaun's Red Lustre crater glaze (can be pulled at 980-1000c or 1030c)
Gerstley Borate frit 38
Copper oxide red or black 10
Red iron oxide 10
Silicon carbide fine mesh between 4 and 10 percent.
Stoneware crater and volcanic glazes
In recent years I've been developing high fired oxidised crater glazes. It's certainly a lot less back breaking than a day's raku firing. Like the raku variant – these can sometimes be abraided to reveal fascinating cratered areas. They often work well with multiple firings, you can abraid areas and re-apply glaze on areas which need it. (The pots need warming to get the glaze to adhere well. ) This method of re-firing can produce really interesting textured surfaces. This intervention with the pot- mimics the “hands-on approach” of Raku and is well worth a try! I have even re-fired stoneware crater pots with a thinly applied raku glaze and raku fired them. There is no end to experimentation with these glaze textures.....Recipes to follow!