Wednesday, March 11, 2009

Copper reds, tenmoku and oil spot glazes

Potteries and porcelains in these glazes can be stunning, and deliciously vulgar. At least, I think so. I really like them.

Copper reds:
Three glazes are produced with oxides in thick glaze mixtures. What else they have in common is that the kiln should be dedicated only to that glaze for that firing. This is because firing is unique to the effect: reducing the kiln early in the firing makes beautiful copper reds, and having only copper oxide glazes in the kiln at that time will have a deeper red than mixed firing because the copper atoms will "fertilize" the other pots.
Whatever the recipe, all copper glazes have feldspar, silica, copper carbonate, tin oxide, and iron oxide.

Tenmoku is a dark glaze with subtle spotting that has disipated slightly. It contains feldspar , limestone , and iron oxide. The name tenmoku comes from 天目 (Tien Mok, Tian Mu) which means 'heaven eye'. The Japanese use it a lot in teabowls. The darkness of the glaze depends on the cooling of the kiln.They can be very different - long firing and an iron-rich clay, as well as a firing-down of the kiln, will keep the glaze molten longer, and let the iron crystalize finely in the surface of the glaze.

Oil spot:
Oil spot glazes are stiff glazes containing red iron oxide and magnesium oxide. The glaze is applied thickly and fired in an oxidation atmosphere. Oxidation is essential to create the oil spots.The red iron oxide molecule will release an oxygen atom at around 2250° F (1232° C) and become black iron oxide. The oxygen heads to the surface of the glaze, pulling a trace of iron along. The oxygen leaves, the iron remains on the surface, creating the oil spot effect. Oil-spot glaze contains a high amount of feldspar, often over 50%. There is also magnesium oxide (dolomite or talc), and from 6.0 to 8.5% red iron oxide.
The thicker the glaze mixture is applied to the pot, the larger the resulting spots. If the glaze is to be applied really heavily, more feldspar is needed to stiffen it, so that it doesn't run too much and fuse the pot to the kiln shelf. But that is usually not a problem. If the firing is very hot, the glaze may run enough to pull the oil spots into the hare's fur effect so prized by Sung dynasty potters.


Oxidation firing: If there is more than enough oxygen present to burn the fuel completely, especially during the last stage of temperature increase, then the firing is an oxidation firing. The metals in the glaze mixtures will convert to fully oxidized, the colors will be more uniform. This is best done with an electric kiln, to avoid the fuel drinking up the oxygen. And by having fresh air coming into the kiln.


GRANT!PATEL! said...

Fascinating. I myself have no affection for the oilspot glazes, though I grant that they can indeed be striking. The brilliant reds and oxbloods are more design element than decorative object - good as part of the background, not as objetdart.
But to each his own.

---Grant Patel

GRANT!PATEL! said...

Or her own.

--Grant Patel

GRANT!PATEL! said...

Plurals and non-genderspecific third persons also good.

Whatever the own is.

---Grant Patel

The back of the hill said...

I have some splendid examples of oil-spot glaze, as well as a number of items that fall into the copper oxide category.

Some of the bowls I acquired from Lin Xin Zhuan are rabbitsfurry, others more tenmoku. The period a couple of years ago when he was strongly influenced by both the Palace Collection (National Museum, Taipei) and the Korean glazes from five and six centuries ago, yielded some lovely experimentals. At that time his sense of proportion was also different, consequently even his plain pieces are a joy to handle.

GRANT!PATEL! said...

Glazed pottery, Both, in lieu of glazed undergarments and grazing hands?

I suspected as much. God you're an utter pervert. Very admirable.

---Alphonso Dingus

The back of the hill said...

Thank you for admiring me. Although I am baffled as to your stated reason - the intersection of my underwear and my pottery collection in your mind is utterly daft.
But yes, Grantipoo, go ahead and admire me.

The back of the hill said...

Is it good for you?

GRANT!PATEL! said...

Ooooh, your silken smooth glazes, big boy!

I'm melting, I'm melting!

At approximately 1600 degrees, of course.

I'm vitrifying, I'm vitrifying!

---Gritty Claybody


You're turning to stone?


Shoot, I should've signed myself as 'Medusa' under that last comment.