Post-Fired Reduction Method

admin | Glaze Research, Kilns | Saturday, September 1st, 2007

I have experimented with reduction atmospheres since school, but it was friend and master crystalline potter Peter Ilsley who started me on his own method during a workshop in Palm Springs, CA.
Since then, I’ve developed my own techniques -but I owe much to this wonderful artist for inspiring me and pointing me in several right directions.

In this case, post-firing reduction defines a second firing used to bring the glaze up to a lower temperature (between 1350-1750°F) than my initial crystalline firings (^11-12). This temperature does not induce a complete melt, but “softens” the glaze. At critical temperatures, the gas/air mixture is adjusted to create an oxygen-reduced atmosphere. The resulting Carbon Monoxide is hungry for oxygen and takes it from the glaze matrix. This thermal/chemical shift alters the metallic coloring oxides in the glaze and changes the color.
A strike firing denotes a similar technique, with the name deriving from a glass-blowing term. The glaze is reheated to a certain point in a neutral or low reduction atmosphere. This can cause subtle changes in the color, sometimes creating iridescent qualities.

For both of these firings, I employ a kiln that I built on site at Wiseman Ceramics Studio. It consists of an old electric kiln, turned on it’s side, to which a burner, chimney, and damper was added.

Post Fire reduction kiln by Jesse Hull

I have seen potters try similar approaches with old electric kiln shells, but all those were made into updraft kilns. I’ve always found updrafts to be inefficient, uneven, and provide spotty reduction effects. In order to get the atmosphere to move both around and through the work, this kiln was designed to act as a combination cross/downdraft. It works remarkably well!

Post Fire reduction kiln by Jesse Hull

Post Fire reduction kiln by Jesse Hull Post Fire reduction kiln by Jesse Hull

 

To be honest, I built this kiln for the fun of it, in wait for the new Geil JH-10 … and, I’ve got to admit that at first it seemed a bit hokey. But the results that come out of it continue to amaze and inspire me onward:

Post Fire reduction effects by Jesse Hull Post Fire reduction effects by Jesse Hull

Post Fire reduction effects by Jesse Hull Post Fire reduction effects by Jesse Hull

Post Fire reduction effects by Jesse Hull Post Fire reduction effects by Jesse Hull

Post Fire reduction effects by Jesse Hull Crystalline Glaze Post Fire reduction effect: Gold & Silver

 

So I’ve been firing in it pretty steady for about 2 years now, and made some modifications to the kiln this past summer.

I learned to fire electric and larger gas kilns manually in school, so although I look forward to exploring glazes with the oxy-probe automated Geil, I have to say that I really value the experience gained by firing a manual reduction kiln in achieving these effects.

 

Related Links:

Electric Kiln Oil-drip Reduction

Jesse’s Crystalline “Gold” Silver Nitrate Glaze

Using Silver (Ag) as a colorant in a ceramic glaze.

 

 

9 Comments »

  1. Hi Jesse… you site is inspiring. I have too many questions to put in one post so I’ll stay on topic with just this. I have been doing electric reduction and have had many great copper red pots come out…now I ‘m working with Phil Hamling
    doing post fire reduction on his cone 11 crystal pots. I don’t use any technology other than a kiln sitter.I judge the reduction by eye but because of the fact that I use wood as the reducing agent I find that my small test kiln is playing tricks on me . The first test was posted on the glaze forum by Phil, the second came out totally different. I was encouraged to see that you fired, early out, manually. I hoped that by watching many firings, studying the flame ,controlling the rate of fuel feed, and adjusting to get a steady back pressure that I could develop a intuitive knowledge of the process that eliminated the need for other gadgets.
    This is the way I believe potters in the past did it…low tech…heavy on intuition and just getting to know by trial and error the subtle ways of the beast.
    If you could throw a little light on the visual indicators you’ve used I would greatly appreciate any help.
    thanks again for all your inspiring works
    David T.

    ______________________________________________________

    Jesse Hull says:

    Thank you David,

    Your own “smoked” work (as Phil calls it) looks really nice… that recent piece on Phil’s site is a pretty brilliant red.

    I have tried using wood in post-firings… but I was usually only able to get the heaviest levels of reduction with it. The more subtle iridescent colors I aim for, as with the redn’d rainbow glaze are lost, replaced with an overall blood red.
    This past summer (after returning from South Korea) I did some complete ^9 wood firings in the kiln pictured on this page. The first attempt was surprisingly good… the second not so much. Because I don’t actually have a firebox to work with, the entire firing was a constant process of adding wood with little down time, and I ended up having to rebuild my chimney (stack) after the 2nd run.
    I do understand that what you’re speaking of in terms of using wood is different, but I thought the above might be interesting to you nonetheless.

    As you mentioned, getting the back pressure is key …just running excessive amounts of fuel/combustibles through a kiln won’t do much, even if you’ve got flame and smoke pouring out of the exhaust. Then there’s the thought that if you’re actually producing smoke, then your reduction effects suffer.
    You mentioned a kiln sitter… I also fire to a cone, although of the upright self-supporting variety, using a 2″ spy hole that I cut into the front door.

    I actually did a full slide presentation at Krystallos 2007 on the creating (and later amendments) of my post-fire kiln, also covering how I fire and even adjust the kiln for barometric pressure, etc prior to each attempt.
    Time is short -but at some point, I do plan on putting all of the images and info from that presentation up on the site here.

    ~jesse.

    Comment by David Turner — March 16, 2008 @ 8:23 am

  2. Well, I am bit by the post-fire bug. But as always realized my learning curve is very steep. Any help would be really appreciated (I have tried to do my homework first and have read every reduction post a couple times). Still at a loss!!

    1. After two firings, I can not figure out how to reduce my Alpine kiln temperature while reducing at the same time. Finally gave up after firing to 1500 and dropping only to 1380. I turned the kiln off and blocked all sources of oxygen. At the end, before I shut down the kiln, I had the blower off, all primary air ports blocked, gas at 0.2, and damper at 3/4″. How in the world do you drop temp and reduce at the same time!! Is it possible in a gas kiln?

    2. I used an oxygen sensor that Terry designed using an auto sensor. It worked great, but the only time I saw a reduction range on the voltmeter was when I had a small red flame with a little smoke. Does this make sense? I am use to a strong orange flame when reducing in my cone 9 firings.

    3. Had 10 pots in this firing. The attached picture is the best I received. Most of the pots were gun metal (they had over 4% red copper oxide alone or silica carbide or ilmenite in with the copper). I torched all these to get some great raku-like crystals, but really not what I was going for.

    4. I think my gas kiln is far too large to ruin so many pots at one time! Probably move onto a garbage can or fiber kiln of some kind. Any thoughts on which design you would try first?

    The experimenting continues and I am having a great time. I think I would be pulling my hair out if my livelihood depended on these crystalline glazes. So much to learn, so little time! Thanks ahead for your help.

    Comment by Anne Gary — March 20, 2008 @ 10:43 am

  3. Anne,
    It’s actually normal to see a small amount of black smoke/flame when I go for good reduction…
    Maybe I could do without it, but it works for me.
    Reducing down is a learned deal… and from one kiln to another, there just isn’t a direct translation. Usually it’s the gas steady or slightly down, with damper in. Or on rare occasions it can actually be the fuel going up a bit. To compound things even more, I must fire differently during dry cold nights than on humid warmer days -and varying levels in between.

    If your kiln is large and heavy with insulation, it will cause problems. Along those same lines, a kiln that will reduce down nicely from higher temps, may not do so well from a temperature 700-800 degrees cooler.
    I remember Diane Creber telling me last year about how much of a chore it was trying to reduce down from 1500F in her larger gas kiln. It may be that even if you shut off the fuel completely at that temp, your pyrometer would drop slowly –so by adding fuel, well…
    But hey –I’m not saying it’s impossible!

    When I speak of timing how long it takes for the smoke from a scrap of oily rag to travel from the burner to the end of the stack, most people just return a blank stare.
    But the rate at which your fuel is traveling through the kiln matters significantly, and the rate achieved at 1500-1100F will be different from that at ^9/10.
    It’s not that I can say: ok, it takes x# of seconds for the atmosphere to move through my kiln and I get great reduction, thus giving you the info that you need to do the same. My kiln is built different, having twice the proportional distance for the flame to travel when compared to other downdraft designs. This same design might not work well if it were larger –but kilns built, tested, and modified for specific applications will obviously hold a much greater chance at succeeding.

    So, in terms of choosing/building a kiln for post-fire reduction, I would suggest some sort of lightly insulated, small chambered downdraft… Back pressure can be achieved in most any type of kiln by using a damper, but there’s more than just back pressure at work. Terry and I were just talking a few weeks ago about the benefits of the fuel/flame traveling a greater distance before reaching the oxy-probe/sensor (and more importantly, the glazed work).

    I know this reply may seem frustrating –and my first several firings were exactly that!
    If I were you, I wouldn’t waste any more good work. Throw some basic cylinders about 8-10″ tall, glaze and fire them in with your crystalline firings. Use a glaze with 1-2% CuCO3, and practice post-firing with these. They’ll tell you what kind of reduction your getting, or if you’re getting any at all.
    Especially if you’ve got a larger kiln, the glazes will be more honest than a statically positioned oxygen sensor. Do keep notes of your sensor’s readings however, as the numbers will make sense later.

    Comment by Jesse Hull — March 20, 2008 @ 10:44 am

  4. Jesse,
    Thank you for all the great info. I guess I did not realize how hard this
    would be. When we were in Florida at John’s workshop, we had no luck
    reducing down either. So I think it is best for me to go to a smaller kiln
    with fewer pots!

    I looked at your site. I really like the kiln you created and have access
    to a great deal on a used kiln.

    It looks like you are using gas, could you use propane in this design?
    Would you be willing to share info on how you created this. I would like to
    try to implement your design or something like it so I do not have to deal
    with a fiber kiln. I hate working with fiber. Plus, I have no idea how to
    create a kiln! But, I totally understand if you do not want to share the
    info. Would love one of the Geil kilns, but way out of my price range.

    I will start experimenting with the recipes. Thank you for the starting
    point. My glazes either had too much copper or not enough. This should
    help.

    After thinking through this and realizing how little I know about building kilns, I have asked a person in Chicago to help me. I really need to assemble something portable so I will not be able create something like your kiln.

    If you had to pick would you use a old electric kiln (use it upright) 28 X 21.5. and convert it to use propane or would you build a fiber kiln. I am concerned that the electric kiln will not cool during reduction just like my problem with the Alpine kiln.

    I can purchase the electric for about $200!

    Thank you for the help,

    Anne

    Looking forward to seeing some copper reds one day! It is a fun journey,
    keeps me challenged.

    Comment by Anne Gary — March 20, 2008 @ 10:46 am

  5. In your case then, I’d go with a fiber kiln for two reasons:
    1- You need portability
    2- I’m not partial to updraft kiln designs… Terry Fallon’s design is the exception, but he’s introducing rather precise amounts of fuel and exhausting it out of a very small opening at the top.
    This allows for the necessary “backdraft” effect.

    If you don’t want brick, then I’d build something like the kiln that Peter Ilsley described at the workshop in Palm Springs. Basically, it’s a square box made of wide mesh steel screen and kiln fiber. At this point, you’ll at least have more of a cross draft kiln to work with.
    Holly McKeen built one like this, so you could talk more about this with her.

    I personally hate dealing with kiln fiber, so I would suggest that you at least consider the magnesium silicate fiber (Isofrax, Unifrax). It is supposed to be a More lung safe (”low lung biopersistence”) alternative to the nastier ceramic fiber that’s more readily available.

    After saying all that, a trash can lined with the same fiber above would be by far the most portable. So, if you want to practice the updraft approach, give it a go!

    Let me know how things turn out…

    ~jesse.

    Comment by Jesse Hull — March 20, 2008 @ 10:47 am

  6. http://picasaweb.google.com/lh/photo/yDPipEtWiRf_0ey24q5Qqg

    Jesse:
    I stumbled onto this site and then this page and I’m astounded. The kiln modification you show above is the perfect solution to my current situation: my old, cranky cylindrical updraft gas kiln needs a rebuid of the burner system and support deck, it’s too hard for me to load (from above and unassisted) anymore and I’m reluctant to invest in a front loading downdraft (Geil) in the current economic times.

    I recently returned home after several years in Australia on a work assignment and I am desperate to resume firing at home, so I had told myself I’d be contented with raku until the downdraft seemed like a reasonable investment. If I modified my old Aim kiln in the manner you’ve shown I could have an intermediate high fire solution until the downdraft solution was do-able.

    Although I can see from the photos how some of the conversion was managed, I’d appreciate the opportunity to discuss via email some of the issues with my kiln (pictured in the link above): it’s bigger, was designed for use with 3 burners to reach 180,000 Btu for C10 firing and has burner ports in the bottom and a damper port on the lid that will need to be filled in and then re-modified, I presume. Would you be willing to discuss these issues with me outside this comment? Please let me know.

    So grateful to have seen this possibility —
    Jeanean Slamen

    Comment by Jeanean Slamen — November 29, 2008 @ 2:53 pm

  7. HI Jeanean,
    I’m a bit busy (these days especially, with winter coming on); however, as long as you’ll allow for some delay in my responses, I’ll be happy to input what I can.
    Aside from adding an occasional patch of refractory mortar here and there, the kiln pictured above looks great, and is still going strong!
    -Having said that, although I have completed firings to ^10-11, I’m typically only going to the 1500-2000F range for post-fired reduction work.

    My initial concern on the higher temps would not be so much with the kiln, but in the chimney (stack). I’ve seen people use heavy gauge metal drainage pipes for wood kilns going as high as ^14, so that may be an option.

    ~jesse.

    Comment by admin — November 30, 2008 @ 9:13 am

  8. Jesse

    Very nice.

    Dale Ferguson

    Comment by Dale Ferguson — May 17, 2012 @ 2:26 pm

  9. I am curious to know if your post-firing reduced glazes reoxidize over time, in the way that post-firing reduced rake glazes are known to do.

    Harriet, with pieces as old as 7 years old and some of them even being displayed in daily sunlight, I have never experienced any of them re-oxidizing.

    Comment by Harriet Campe — February 10, 2013 @ 9:50 am

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