Oxidation and Reduction Firing Schedules
Linda Arbuckle Course Notes
John Baymore Thoughts
From a thread in Ceramic Arts Daily:
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
Born To Fire, I’m Inspired I want one!
https://www.youtube.com/watch?v=wajoxWN2qzw
Reduction - smooth repeatable results in about eight hours!
https://www.youtube.com/watch?v=Hzrsbf3NZLo&t=59s
Raku, lasers and reduction - fun!
https://www.youtube.com/watch?v=X0UJQH3Er0w
Other References
Reduction fired low-temperature ceramics; D.Dawson, O.Kent; Post-Medieval Archaeology 33 ( 1999), 164-178
<HR>Licence
Tim Thornton Ceramics Technical by Tim Thornton is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
Permissions beyond the scope of this license may be available at https://tim-thornton.com/tikiwiki/tiki-index.php?page=Licence.
If you re-use any material from this web site, you must include an attribution statement as described in the licence, and state if the work has been altered.
You may release work that modifies or builds upon this work only under the same licence (or a compatable one, as defined in the licence).
This licence does not permit you to use this work for commercial purposes, i.e. for commercial advantage or monetary gain. If you wish to use the work in this way, please contact us to discuss it. You will need our prior permission and approval to do this.
Whilst we aim to ensure that all material is accurate at the time of publication, we do not accept responsibility for decisions based on information we make available. No content of this site may be taken to be legal or medical advice.
Please bear in mind that materials on the website are the most up to date versions, but items you have downloaded in the past may not be. The materials are updated from time to time. We recommend that you use the latest versions by referring to those published on our website.
Exclusions
Images – Images are not available for re use under the OGL because in most cases copyright is held by a third party. The Council does not grant anyone any rights to re use images.
Use of logos – The use of Tim Thornton’s logos on any document or in association with any information, signifies that the document or information has been prepared or approved by him. The logos are to be used only when reproducing materials unaltered from the web site, to show their original source. If you wish to use the logo in any other way, please contact us first to obtain permission.
Third party intellectual property/copyright – We do not have the legal authority to grant permission to re use documents where legal rights are owned by third parties.
Applications for re-use
If you wish to make a specific application to re use information which does not conform with the CC BY-NC-SA 4.0 licence terms, please contact us to ask permission and for details of what, if any, additional conditions and/or charges may apply. Your application should be in writing, specifying your name and address and, identifying the information source you want to re-use and the purpose for re-use.
Disclaimer
Whilst every care has been taken in the compilation of this website, and every attempt made to present up-to-date and accurate information, we cannot guarantee that inaccuracies will not occur. We will not be held responsible for any loss, damage or inconvenience caused as a result of any inaccuracy or error within these pages.
Links from our site may lead to pages maintained by other organisations. These are provided purely for your convenience, and do not imply that we endorse or support those organisations, the information on their pages, or their products or services in any way. Additionally, no responsibility is assumed by us for the contents of its pages.