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Proper Reduction Firing Schedule


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I learned back in the day that you do a reduction soak at the end of the firing.

 

I just read a PDF by Linda Arbuckle saying reduction should be done at ^010.

I've also read a similar statement to Linda's on another forum.

 

Is she talking to do the reduction on the way Up or on the way Down?

:huh:

 

"You should do reduction about cone 010, and reduce for 20 †30 mintues. If you begin reducing later then 06, you may have missed reduction and will not get any."

 

http://lindaarbuckle.com/handouts/reduction_fire.pdf

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A reduction soak is not necessary at the end of the firing. If you haven't achieved reduction by then it's too late. For some glazes, however, it helps to do an oxidation soak at the end. I used to do it for my copper reds, as it cleared any muddiness out of the edges where the glaze breaks to white.

 

Get up to body reduction temps- cone 012 to 08- as quickly as you can while not cracking any pots and maintaining evenness. No sense wasting gas with a really slow climb if everything has been bisqued. Put the kiln into reduction at cone 012-08, and stall out the climb. Hold temperature with the reduction for 45 minutes. If you're trying to carbon trap a shino glaze, then make this a heavy smoky reduction. Then put it into a neutral atmosphere and let it climb to cone 10. Stalling it out in reduction will ensure that you get good reduction throughout the kiln, and climbing in a neutral atmosphere will give you an efficient use of gas.

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Ah ha! Thank you very much.

There is more info I've read since posting, that there is a 'body reduction', which means darkening the clay body etc, but I'm interested in reducing the glazes.

I always thought the reds turn red by reducing at the end.

Live and learn...you can teach an old dog new tricks!

 

BTW I'm using ^5-^6 glazes.

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I never soak in reduction (that is to stall out)-my whole life has been in reduction firing . I reduce around 1800 degrees and stay in that until the end (cone 11)Near the end its climbing very slowly.
.When I shut it off it oxidizes immediately (on my oxy probe) and that seems to be enough for great results.
Mark

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I fired ^6 reduction for 20 years when I was teaching after doing ^9-10 for 12 years prior to that. . I did a body reduction for both

firings from about 09-04.  If you are going for copper reds you should take it up in a reduced /neutral atmosphere. 

I reduced towards the ends of the glaze firings around 8-10 or 5-6 depending on which I was firing.

 

Marcia

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Guest JBaymore

There are many variables involved in developing specific cycles for specific effects with specific bodies and glazes..... but in general, the concept of "body reduction" as somehow being totally separate from "glaze reduction" is not quite accurate to what is actually happening. 

 

It is all a "blur" that runs together seamlessly. 

 

If you want to cause reduction effects on compounds in the body, you need to start reduction before the surface of the clay body or the surface of the overlying glaze layer becomes gas impermeable to the two prime reducing agents in firings; carbon monoxide and hydrogen.  Carbon monoxide is the main reducing agent in most of our firings because hydrogen at elevated temperatures is so reactive it usually "finds" available oxygen first in the kiln's gases. 

 

Note that carbon particles are SO large that they are quite non-useful for "reduction" in a kiln.  They don't easily get into the clay or glaze surface.  SO all that smoke some people get on gas kilns..... wasted effort. (Unless you are doing carbon trap shinos! ;) )

 

So the molecular size of the reducing agent getting into the clay is the first question in developing the firing cycle. As the body tightens, it does not allow the gases to get to the compounds below the immediate surface.  The main compound that we use to get "reduction" colorations in our work are iron compounds, reducing the valiancy to the black state from the oxidized red state.

 

FeO is a powerful flux on silica (SiO2) and which also happens to color the resultant glass at the same time.  So as the reduced iron 'bleeds' into the glassy phase of the body it also colors it (grey). This is particularly effective on small hematite nodules....giving lots of reduced iron in a small physical location (iron spotting). 

 

When the SURFACE of the tight body that is NOT under the glaze is then allowed to get in contact again with oxygen as the kiln is shut off and cooling, the surface turns to the reddish-browns we associate with "reduction" firing.  The body, if it is vitrified, is gas impermeable to oxygen ... and the inside it remains a greyish coloration. 

 

Importantly, the diffuse reduced iron compounds in the glassy phase of the body start to also work their way into the melting glaze layer over the already reduced body.  If the glaze surface starts to melt and become gas impermeable to the reducing agents before reduction had occurred, the body under the glaze does not get reduced.  So the interaction of the body with the glaze is less.  Plus the coloration of the body will be base on the red state (or the original state) of the iron compounds. 

 

Once the glaze surface is totally gas impermeable to the reducing agents then no amount of reduction will reduce stuff own inside the glaze layers or the underlying body (except maybe because of the bubbling of the glaze allowing some interior matter to be reduced on the surface). And once reduced inside, when the kiln is shut off and cooling and exposed to oxygen, the glaze surface also, just like the clay surface is oxidized.  Unless you fire down in reduction, or pump the kiln full of an inert gas (industry does this kind of stuff) the whole outside of "reduced" pieces is re-oxidized. 

 

Different glazes and different clay bodies have different firing characteristics.  There is no "one size fits all" firing cycle.  You have to find what gets the best out of your clay and glazes.  And different glazes and bodies often are not getting optimum effects when fired together in the same cycle.  What is great for one may be just OK for others.  We are back to that stock phrase of mine.... "test, test, test".

 

At one point I did some research on one particular celadon glaze on one particular body.  Over a LOT of firings it was determined that reduction at a certain intensity had to occur before cone 04 and after the kiln reached cone 4 it made absolutely no difference how the kiln was fired as to oxidation or reduction levels.  Reduction too low below 04 (cone 012 and down), and the body exhibited a tendency to carbon core and bloat and bleb.  Optimum turned out to be slightly oxidizing or neutral fire up to about cone 06 to 04 to start a light reduction, and that level was maintained at the same level until cone 4.  Then fired in slight oxidation to neutral to the cone 10 end point, then cooled in oxidation.

 

Some thoughts for ya' there.

 

best,

 

.......................john

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Ah ha! Thank you very much.

There is more info I've read since posting, that there is a 'body reduction', which means darkening the clay body etc, but I'm interested in reducing the glazes.

I always thought the reds turn red by reducing at the end.

Live and learn...you can teach an old dog new tricks!

 

BTW I'm using ^5-^6 glazes.

In a copper red firing if you only reduced at the end of the firing you would come out with either clear or greenish celadon like glazes. Copper volatalizes at very low temperatures so if you dont start your reduction early you will just be burning the copper off and sending it out the flu for the remainder of the firing. You need to go into body reduction early (cone 012-08) to form the reds, and maintain either neutral or reducing for the remainder of the firing to enhance and maintain the reds formed early on. You shouldnt really think of "body reduction" as a period where you are darking the clay body specifically. If your pot's cross sections have extremely dark cores you may be over reducing, however one size doesnt fit all. Over reducing can lead to many clay and glaze imperfections. Above, John explains reduction very well. I too reiterate, "..test, test, test.."

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Good input here, and thanks for refreshing my memory and refining my knowledge...or lack there of. ;)

 

Saturday I did the reduction as described for 30 minutes about 1650F-ish. Reduction with flame showing but no black smoke.

Fired slower in general and soaked at the end for 30 minutes and slowed down the cool down as well.

I think I'm getting this kiln dialed.

 

IMG_4511-L.jpg

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From what I can remember operating the gas kiln, we fired the kiln

gradually after soaking overnight. Then around 1650 we turned down the

pressure, dampered down both blowers, closed the flue to a prearranged

mark, and watched the temp gauge making sure temps were rising.

After a couple of hours, we started checking the large witness cones..

Until cone 11 started drooping half way.

 

Sometimes we'd forget and the results were normally good anyway.

See ya,

Alabama

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