First, a brief note on process...
The Japanese, who originated the process, would throw their blazing bowls into a cold water bath. The glaze, subjected to horrendous levels of thermal shock, would crack in all sorts of unpredictable arcs and webs. (Anyone who's used a cold shower therapeutically can probably relate).
The North Americans (Paul Solder, among others) added a reduction step, wherein the wares were placed in a sealed chamber of some type with combustible materials (i.e. sawdust, paper, wood shavings). I sometimes think that shoving a 1800 degree chunk of clay into a newspaper-lined trash can is even more fun than pitching a marshmallow into a campfire, but my seven-year old self might disagree.
The combustibles burn up, sucking oxygen out of the atmosphere in the reduction chamber. Free oxygen in the glaze (and even the pot itself) is mobilized- in the process releasing metallic oxides. Coppers and irons flash and flux over the surface.
It's alchemy... and like a mid-evil scholar trying to stir up a mix of saltpeter and mercury in a kettle (gold!!!) the results can be unpredictable.
1) The firing temperature and the overall amount of heat work absorbed over time by your wares
2) The amount of time a piece is left in the reduction atmosphere
3) The intensity of reduction (how well sealed the container is, how much combustible material it contains)
4) The chemistry and texture of the combustible materials
5) The time lag between removal of the wares from the kiln and the initiation of reduction
6) The phase of the moon
7) The creativity of any songs, liturgical dances, libations, or other propitiations to the Raku demons***
8) The karmic impact of any mosquitoes, grackles, or cats that blunder into the kiln
(***The word 'demon' is used here in the more ambiguous, multi-faceted Japanese sense of the word).
One other note- Raku is not food safe. Aside from pourous vessels and bacterial hitchikers, raku wares don't hold their metals very well. If you want a healthy dose of cobalt or copper oxide in your tea, however, forge ahead. I once made a raky beer stein and used it to drink a pint of Guinness- suffice it to say that my lymph nodes were tingling for hours afterwards.
Anyhow, with those disclaimers out of the way, here are some notes on a few glazes that I use.
One of the goals for many raku artists seems to be a 'copper penny' surface. Bright metallic textures.
Honestly, I'm a little less excited about copper hues than I am about some of the earthy reds you can obtain with copper and iron based glazes. The classic problem with glazes of this type is that the brightest hues often fade (as the glazes are innately low in silica). Tom Buck posted an excellent article in Ceramic Review where he discusses this issue- and how to side-step it.
I've been using his Copper 80/20 recipe quite a bit. It's base appearance (on the right above) tends to be a brick red... but you can also see iridescent, oily textures, as well as mottled lodestone patterns (left, above). It's a great glaze for accents, and very easy to work with in its wet state.
As with all the recipes I'll post, the numbers below are dry weights.
Copper 80/20 #6
Gerstley Borate (99) 67
Frit 3195 17
copper carbonate 10
Iron Oxide Red 5
Crazing, or 'crackle', is a highly undesirable quality in functional wares. Micro-cracks weaken the clay matrix, and serve as a nesting site for e-coli, legionella, anthrax, and other iterations of the creeping crud.
In Raku, however, 'crackle' is a desirable quality, and many glazes seek to maximize the patterns. In Japaneses tea bowls, the tannins in the tea would seep into the cracks, darkening them over time. Many raku glazes seek to evoke this effect (especially as the reduction process naturally blackens the seams).
Ferguson's crackle is a wonderful, reliable glaze, great for background. I've accentuated cracks by various means (spritzing hot pots with mist, waving them in the air at the end of a tongs, blowing compressed air on the hot glaze). The effects are varied, and always tantalizing.
Ferguson's White Crackle
Gerstley Borate (99) 65
Nepheline Syenite 20
Helmer Kaolin 5
2) Turquoise Crackle
This particular vase features a couple glazes I've only started working with recently.
'Turquoise Crackle' is a sandy textured, devilishly simple glaze (only three ingredients). It's a delight to work with though- runs on smooth as silk (unlike many raku glazes, which tend to clump, bleed, and dessicate like frogs in the outback).
Texturally, its oxidized color isn't exactly turquoise- more of soapstone hue... but I like it. It also flames to interesting iterations of rose and sandstone in reduction- see below.
'Higby Lime Green' is the opposite animal, in terms of ease of application. I swear that every brushstroke carries a tang of the outer Kalahari... the stuff absorbs water like a triathlete north of Kona. However, it has a rich, jade-like texture, and some lovely, off-red reduction effects. Again, see below.
Frit 3110 73.7
Gerstley Borate (99) 5.3
Soda Ash 10.5
EP Kaolin 5.3
Copper Carbonate 8
Gerstley Borate (99) 80
Copper carbonate 4
Titanium Dioxide 20
This recipe- like the crackle recipe above- was developed by Tony Ferguson... but I originally looted it off Jeff Guin's website, 'Clean Mud'. (Jeff's a kiln-building clay junkie out of Wisconsin). Thus the name.
This glaze produces a vivid turquoise if applied thickly in oxidation, a robin's egg blue if applied thinly, and all dimensions of red and carnelian when fired in reduction. It's unpredictable and vibrant, just like the process itself.
Frit 3110 100
Tin oxide 3
Copper carbonate 3
'Egyptian Blue' may be the most frustrating glaze I'm working with at the moment... but it's undoubtedly my favorite. It's a weird recipe- more loaded with 'soda ash' (Sodium Oxide) than a Coca Cola truck in a forest fire. Soda ash yields a textured, sand-blasted outer skin. The main colorant- black copper oxide- seems to be capable of yielding every color in the rainbow, conditions dependent. The most common outcome is a mossy green (above left) with crimson flashes appearing at whim (right).
In heavy reduction, brick reds and duns tend to predominate.
1) A reliable yellow crackle glaze (all efforts to this point have been unsatisfactory)
2) An onyx black
3) A glaze that perfectly evokes the Pacific Northwest tide pool (the 'Clean Mud' glaze has potential, but still needs some tweaking.
Still, it's enough of a palate to keep me busy for a long time.