Clean Up After Reduction Firing

[jrgpots]

I have read a number of reduction schedules for downdraft kilns.  Many of the schedules include a "clean up Phase" (to smooth out surface inperfections) at the end of the firing just before the kiln is shut down and closed for cooling.  One schedule for a large kiln had a clean up of 1 hour while a smaller kiln only had a clean up of 30 minutes.

 

SO....I have a 3.5 cubic foot baby downdraft conversion kiln.  How long should my clean up phase last?... 15 minutes?

 

I'm getting excited to try my little kiln in reduction.  It will take me another week before I have enough bisque ware for firing.   Unlike so many of you, I am a "slow poke."

 

Jed

[bciskepottery]

Ken Matsuzaki holds his wood-fired kiln at temperature for several days to get a nice, smooth melt; but that would be overdoing it for your baby downdraft.

 

You may want to try a variety of holds at the end to see how your glazes react, then pick the one that works best. Maybe start with 5 minutes, then 10, then 15 -- or do it in reverse order.

[Callie Beller Diesel]

Wouldn't this depend more on the glazes and clay involved? If it's to fix surface imperfections, why not look into the kiln using welding goggles at the top end, and make sure everything looks smooth?

But a 15 min "soak" at the end couldn't hurt.

[jrgpots]

I'll try the 5 minute first, followed by 10 and 15 minute.  I will write down my proposed schedule tonight and post for review...

 

Once I can replicate my desired firing conditions, I can invite others add a few pieces.  Our studio right now is just cone 6 electric.  I'd love to get others interested in reduction firing. 

 

Jed

[Mark C.]

I fire my reduction kilns(almost weekly) with no clean ups. They oxidize as soon as they shut off. My oxy meter shows this well.

I brick up the ports and let cool slow.

I also usually have a few reds in my kiln and do not want to loose the color.

Wood fires are a bit different all the way around.

Mark

[jrgpots]

So maybe I should fire without clean up and see if the surfaces have any problems. If no problems, then no clean up, thus better reduction in the glazes.  I think my little kiln will cool down rapidly.  Hopefully I can shore up all of the drafts, burner ports, and peep hole enough to hold the reduction. I fear the size of the kiln my work against the maintaining of reduction as it cools.

 

If it does not hold reduction well, would a small stream of CO2 from an outside tank help until the glazes have cooled enough to set the reduction?

 

 

Jed

[neilestrick]

A hold at the end of the firing can serve one of two functions, or both. First, a hold time can help to smooth out any glazes that may have problems such as pinholing. This can be true of any type of firing, whether reduction or oxidation, gas or electric, etc. The second type of hold that many people do at the end of a reduction firing is a period of heavy oxidation. This can affect the color of some glazes. I used to do this for my copper red glazes, since without it the white rims were often more grey than white. I stalled out the kiln for 15-20 minutes with lots of air moving through, then shut it down. Neither type of hold/cleanup is necessary unless your glazes warrant it.

[Mark C.]

My car kiln is soft brick and (fiber Door) and outside its hard brick which keeps the heat after kiln is off. I get crystal development vary easy.

Back when I smoked stoneware more I did a few mintutes of clear but now my all Porcelain loads seem to work fine without that. I am not in super heavy reduction ever now. I fire now with the oxy probe and what I can tell you is the damper is way more sensitive than most can imagine. A little in or out (less than 1/8 inch) can do alot when its just right.

You could wrap that kiln in more fiber to aid the cool down speed.

Mark

[Guest JBaymore]

There are so many misunderstood firingt ideas and "myths" around the firing of kilns it is amazing.  And stuff gets passed on to newer potters as "rules" and "truth" and so it goes on and on and on...........

 

Neil and mark have it about covered.

 

And think through the concept of what the "cl;ean up" (soak) is doing.  It is about something to do with the clay and/or glaze function.  The pot does not know if it is loaded into a big kiln or a little kiln.  IF (and that ios a LARGE if.......) a soak )clean up) is even necessary fro some reason....... if it needs 1 hour to do that in a BIG kiln......... it needs that same hour in a little kiln.

 

Now if you are talking about evening out an uneven firing to "fix" the clay/glaze... then the size of the kiln matters.  But othrwise.......  nope.

 

best,

 

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

[jrgpots]

Ok,I want to give a summary of what I understand to check if it is correct.

1. The "clean up" is another term for soaking.

2. A soak is a time of oxidation before the kilh is shut

3. A soak would hrlp if I want to "soften" the reduction in order to give surface oxidation to thin glaze areas or edges of clay.

4. A soak would help my glaze/clay body if I had larger particle size in my glaze/clay. For example, if I want to add pebbles of granite to my clay body, I may need to soak to allow the pebbles to melt.

5. A soak can help hide/correct my uneven firing technique, allowing the kiln to equally redistribute its heat.

 

6. If I don't need any of the above, I don't need a soak.

7. And finally, if a soak of 2 hours is needed in a big kiln, it is also heeded for the same time in a small kiln.

 

Is that right? Do I have it yet?

 

Now a question.....When and why is a preheat used?

 

Jed

[Guest JBaymore]

Ok,I want to give a summary of what I understand to check if it is correct.

1. The "clean up" is another term for soaking.

2. A soak is a time of oxidation before the kilh is shut

3. A soak would hrlp if I want to "soften" the reduction in order to give surface oxidation to thin glaze areas or edges of clay.

4. A soak would help my glaze/clay body if I had larger particle size in my glaze/clay. For example, if I want to add pebbles of granite to my clay body, I may need to soak to allow the pebbles to melt.

5. A soak can help hide/correct my uneven firing technique, allowing the kiln to equally redistribute its heat.

 

6. If I don't need any of the above, I don't need a soak.

7. And finally, if a soak of 2 hours is needed in a big kiln, it is also heeded for the same time in a small kiln.

 

Is that right? Do I have it yet?

 

Now a question.....When and why is a preheat used?

 

Jed

 

 #1 - Yes... sort of.  "Clean Up" means whatever the speaker/writer thinks it means.  Some might consider it to be a soak held in in oxidation.

 

#2 - No.  A soak can be in oxidation, neutral, or reduction atmosphere.  A "soak" is just a holding period at about the same temperature -only slight variation if any.  (The heatwork is still increasing.)

 

3# - No.  Unless you have a very special (and unusual) kiln..... the kiln is going into oxidation (if it was not already there) the instant you cut off any possible fuel source. Unless you alter it, it stays in oxidation throuighout the cooling cycle.  The surfaces of ALL glazes will typically be oxidized (that is why "reduction fired" stoneware looks brown.... the surface is re-oxidized).

 

#4 - Yes.  This gives the time for heatwork to accumulate and time for chemical reactions to complete.  You can often have a similar effect by firing to a higher cone (more heatwork).....but not always.  Sometimes large rocks take a LOT of heatwork to melt.

 

#5 - Yes.  Basic law of thermodynamics...... heat energy ALWAYS flows from areas of higher concentration to areas of lower concentration.  (Water does not flow uphill.) If heat energy input and heat losses are stabilized (soak), then the hot parts will get cooler as the energy moves to the cooler parts, and they then get hotter.  (Temperature is the result of the application of heat energy to a substance.)

 

#6 - Yes.

 

#7 - Yes.... IF that soak is needed for something other than evening out the thermal lag or unevenness in the big kiln.

 

A lot of the terms that potters use are NOT "legally" defined nor universally understood.  So "preheat" could mean most anything.  For some it means lighting a pilot burner on a sealed load of ware and running it overnight.  For some it means turning on the bottom bank of elements on an electric kiln overnight with the lid mostly open.  For some it means burning a wood fire in the damp kiln to dry it out before loading the wares.  Generally it means a very low and slow application of heat energy in the kiln.  Only typically needed if the kiln is wet or if the wares are wet.  Can waster HUGE amounts of energy.  Often is done totally unnecessarily.

 

best,

 

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

[jrgpots]

WARNING, long post

 

Glaze firing:

 

1. Set burner pressure to 2psi, close damper and lid, plug spy holes.

2. Light burners and allow them to warm up 15 - 30 min.

3. Turn on blowers. This should create back pressure in the chamber.

4. Allow to burn 30 min, then open the damper by 1/4" at a time until the acrid smell of gas is gone (kiln now in oxidation).

5. Continue this slow ramp until I see a dull red glow.

6. Watch for cone 010 to bend.

7. Close damper, increase burner pressure to 5 psi, turn off blower and restrict primary airflow.

8. Check for lapping flames at top peep hole.

9. Keep in reduction x 1 hr.

10. Turn blower back on, open primary airflow, open damper slowly by 1/4" at atime until no acrid smell is noted.

11. Watch for target cone (cone 6 or 10) to bend. (4-6 hrs

12. Soak for 1 hour if indicated (from above thread).

13. Close damper, open passive damper, turn off burners,close burner ports and wait until temp is below 200° F.

 

 

Critique/suggestions requested......

 

Jed

[Mark C.]

Maybe I took the title wrong on this post

Its ok to clean up  the studio or house or wherever after a reduction fire

 

 

Kidding aside why not reduce while blower is on ?

All you need do is oxidze up to cone 08 or so and then do a body reduction and then keep in mild reduction until your end point cone falls.

Pretty simple really
you can keep the blower on that whole time.

If you fire at night you can see the kiln better if you are new to reduction.

Mark

[jrgpots]

My studio could stand a strong clean up...how did you know?

 

So on step 7, I don't have to turn off the blowers, just place the kiln in deep reduction for 1 hr, then mild redution till target cone is bent. Got it.

 

I'm new to everything. What would I be watching for in a night firing?

 

Jed

[Mark C.]

My studio could stand a strong clean up...how did you know?

 

So on step 7, I don't have to turn off the blowers, just place the kiln in deep reduction for 1 hr, then mild redution till target cone is bent. Got it.

 

I'm new to everything. What would I be watching for in a night firing?

 

Jed

Take it easy on deep reduction-1 hour is a a bit long.

I usually judge my reduction by bottom spy plug

The mild occasonal flame lick is seen more easy at night.

Mark

[Babs]

In another layman's terms. the oxygen hungry flame licks out of the spy hole to feast on the oxygen  in the outside atmosphere. not very visible during the day.