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tomhumf

Advice on reduction / firing cycle for speckled clay

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I've been struggling with this glaze for a while, I really like it when it works... 

It's a speckled clay with white glaze I've developed on all these photos, fired in a tiny gas kiln. 

I like the mug on the right - the glaze breaks on the rim, handle and throwing lines, and there's a bit of speckle showing. I don't like the left one at all, the glaze is melted but very boring. 

I like the yarn bowl results - similar to the right mug. 

These have all been fired with between 30 mins and 1 hour reduction at around 1500 F - then completing the firing in a neutral atmosphere. I've played around with a lot of schedules. Some just holding on reduction at around that temp, some climbing through it in reduction.

I can't seem to pin down what temp or amount / length of reduction is giving this look. Perhaps it's more to do with final temperature though I've aimed to fire everything to cone 7 consistently. 

The plate was done in the one firing where I began reduction at 1800F and stayed in reduction until the end of firing. I don't like this look as much. Although the speckle is very pronounced I don't like how the glaze has lost its whiteness. ( I think I may have splattered iron on the surface and painted the rim with iron on this one, so not a perfect comparison) 

I've started making better notes in my firings, but maybe someone might know if the look I'm after is to do with reduction or simply temperature? Or any other hints :)

Thanks!

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Edited by tomhumf

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My first impression is the main difference among the four items is glaze application , especially differences in glaze application thickness.  Measure the thickness by scratching through the glaze, after the sheen goes away, with a sharp square corner of a stick or metal scraper.  The width of the “V” scratches is 2x the glaze thickness.  Reduction amount and timing is also an important variable; thickness can overpower reduction.  

I agree the left cup just sits there; the others have a stronger voice.

LT

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I can see how you think glaze thickness is the issue. I have keep glaze the same, and same dipping time for ages while trying different firing cycles. 

I thought the thickness might be an issue myself and thinned the glaze a little for the one on left. Not sure if that means thicker glaze would be better....but I've had plenty of bad results with the previously thicker glaze too. 

I will try take a pic before i go on holiday of the bases, they are very similar. The right one  is very slightly darker but not really dark like on the plate base. 

 

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So I am going with it’s likely more reduced and the iron that is reducing acts more as a flux and makes these break a bit more pronounced. So, if you have an oxygen probe this becomes more program oriented until you dial in the amount of reduction you like. Body reduction (starting at 1500 f) is good for several reasons  so I would try to dial in the right amount of body reduction to get to your preferred look. If you have an O2 probe this is easy, if not it is a bit tougher to repeat.

Of course there are many other things that can influence this as well, but my guess at this point would be how much this is reduced.

here is a simple video we created for our students which sort of gives an idea of what they did to  learn basic reduction and get some consistency in their reduction firings. Once they figured this out then they make small adjustments to the schedule and learn what the effect is. They have this O2 probe though so a bit unfair and your variation can still be due to other reasons.

 

 

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If we can agree that the glaze is all the same thickness in all the pots -then its the reduction schedule if they are all fired to the same cone end point.

The difference in the plate and the other pots you like is a very thin point. The all white mug looks unreduced for sure.

Reduction fired pots are always a bit different looking-thats why I embrace that process for 45 years. You will always have a few that look less that stellar in reduction.

The placement may be more of an issue where they are in that kiln than some other factors. We all are assumming that that white mug is unreduced but it may also been in a cold spot or a less reduced spot as most rection kilns have those areas. 

I just do not have enough info to make that call but you could think about those things as you know the details I do not.

Edited by Mark C.

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More reduction/speckles will equal a less white glaze. That's the tradeoff. Finding the sweet spot and replicating it throughout the kiln and from firing to firing will be very difficult. If you want speckles and whiteness, and the same results every time, fire them in an electric kiln with a speckled clay body meant for oxidation- one that uses manganese for the speckles rather than iron.

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On 7/26/2019 at 12:06 AM, Callie Beller Diesel said:

@tomhumf do you know if the speckle in your clay comes from iron or manganese?

It's iron

On 7/26/2019 at 12:12 AM, Bill Kielb said:

So I am going with it’s likely more reduced and the iron that is reducing acts more as a flux and makes these break a bit more pronounced. So, if you have an oxygen probe this becomes more program oriented until you dial in the amount of reduction you like. Body reduction (starting at 1500 f) is good for several reasons  so I would try to dial in the right amount of body reduction to get to your preferred look. If you have an O2 probe this is easy, if not it is a bit tougher to repeat.

Of course there are many other things that can influence this as well, but my guess at this point would be how much this is reduced.

here is a simple video we created for our students which sort of gives an idea of what they did to  learn basic reduction and get some consistency in their reduction firings. Once they figured this out then they make small adjustments to the schedule and learn what the effect is. They have this O2 probe though so a bit unfair and your variation can still be due to other reasons.

 

 

Great video, is the longer version online somewhere? Id be interested to see it if so. I have a couple of questions... the attached photo is of my latest firing. Same glaze but a bit more frit this time, all the mugs were in same firing. 

I'm wondering if the differences in look is because I didn't ' fill the kiln and not have any dead spots' like you say. In this firing I reduced between 1600F and 1900F, and was in oxidation ( I think ) after that up to cone 7. 

I will probably try and follow your schedule next time, and maybe reduce more in your heavy reduction section. 

I'm a bit unsure of how best to get the temp to climb while under heavy reduction. My method of reducing my little electric kiln conversion is to shut off the chimney to about 1 inch gap. This creates  a reduction flame out of chimney and peeps but makes it hard to gain temp - I understand this is because its burning inefficiently.  But if I increase the gas burner power it's just going to burn more inefficiently? ...but if i reduce gas it will go back to oxidation

I guess I'm missing some of the control of larger kilns, and I don't really understand the terms primary and secondary air. From what I understand they the air mixed at burner port (primary) and air available at chimney (secondary) ?

Wish I could get an oxy probe but no budget for that unfortunately. 

What happens if you reduce under 1400? Not that I want to but seems like something really bad would happen...? 

@Mark C. I understand your point about cold spots / oxidised spots. Hopefully I will be able to minimise them in some way, I guess a lot is down to kiln design which probably isn't great in my case. 

@neilestrick I'm stuck with gas firing really at the moment. I understand every pot won't be the same. I'm happy with 75% of the pots out of this lot, just wish I could prevent the really boring white ones ( like 3rd right and 3rd left) in the photo. 

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Edited by tomhumf

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19 hours ago, tomhumf said:

I'm wondering if the differences in look is because I didn't ' fill the kiln and not have any dead spots' like you say. In this firing I reduced between 1600F and 1900F, and was in oxidation ( I think ) after that up to cone 7. 

I will probably try and follow your schedule next time, and maybe reduce more in your heavy reduction section. 

I'm a bit unsure of how best to get the temp to climb while under heavy reduction. My method of reducing my little electric kiln conversion is to shut off the chimney to about 1 inch gap. This creates  a reduction flame out of chimney and peeps but makes it hard to gain temp - I understand this is because its burning inefficiently.  But if I increase the gas burner power it's just going to burn more inefficiently? ...but if i reduce gas it will go back to oxidation

I guess I'm missing some of the control of larger kilns, and I don't really understand the terms primary and secondary air. From what I understand they the air mixed at burner port (primary) and air available at chimney (secondary) ?. 

What happens if you reduce under 1400? Not that I want to but seems like something really bad would happen...?  

 

Experimentation is key however the heavy body reduction used in that schedule is beneficial for many reasons most of all for body reduction before stuff starts to melt. As things melt, much harder to reduce the body. So from that perspective early is better than later. As far as getting a temperature rise, it’s difficult under heavy reduction to have a whole bunch of heating energy in the flame.  As your kiln heats up, the hotter it is the greater the losses so probably harder for your kiln / burner arrangement to make final temperature, especially with a reduction flame.

Another reason to do your heavy (Claybody) reduction at an appropriate low temperature versus higher!

So let’s do a theoretical flame thing, total made up numbers. Let’s say a heavy reducing flame is about 1/3 the heat energy of an oxidizing flame and medium reduction, 1/2 the heat energy. As we move from heavy to medium to light we add more oxygen which gives us more heat efficiency per cubic foot of gas.

So what, you say! Well we get about 1000 Btuh for natural gas and 2000 btuh for propane for every cubic foot of gas we put in the kiln, fully oxidized. So here is the trick, if we need more energy we simply need more gas. So turning up the gas pressure and maintaining a set reduction amount constant will increase the energy and therefore the heating rate.

In the video, we move from a relatively low pressure of 1-2” to 4.5” or more at body reduction time to keep the heating rate up and ensure there are no dead spots in that old updraft Alpine. Trial and error was used to figure that out for that kiln.

For greatest efficiency we try and close the damper just enough to slightly pressurize the kiln then back off for a neutral kiln in oxidizing mode. This is common for updraft and downdraft kilns and gives the user a way to set the most efficient point for the dampers saving fuel and minimizing secondary air infiltration.

The point of all this? Many who reduce will simply set their neutral damper early at say 1300-1400 degrees and push themselves into reduction later by increasing the gas pressure only. This gives them more energy, and fills their kiln pretty effectively and often requires they open their damper ever so slightly to get their reduction amount decreased as desired. By opening their damper they are actually letting just the right amount of secondary air to leak in, which gives them their desired reduction and .............. more heat energy ! It does however let more heat out so there is a limit to doing this.

lots of trial and error here, but is an easier way for most to keep their firings straight and consistent.  In the end they often back off the gas slowly to lower their reduction which will add efficiency back in and maintain some reduction to the end of firing. A nice test for reduction is to ensure a jet always exists at the top and bottom site ports. The jet will disappear at the bottom first if there is not considerable reduction and expansion of the unburned fuel to keep it poking out the lower port.

The schedule in the video stays in some level of reduction throughout which is a method. There are many methods and most have been developed to work with the potters glazes. Unfortunately, trial and error is king, along with keen observation. Experienced reduction potters gather a bunch of experience often using the science and combustion theory as a way to help formulate and test their firings.

Do not body reduce too soon, it’s usual to wait until 1500 degrees.  One reason why, the self ignition temperature of gas is generally  approximately 1300 degrees so be well above this for safety to ensure no excess unburned  gas can accumulate and go boom! 

  • So don’t reduce till 1500 or above
  • minimize infiltration of secondary air as practical
  • research reduction glazes ...... copper red: requires about 0.2 - 0.5% copper and three times the tin (and a steady reduction atmosphere throughout the firing) to come out bright and clear.
  • Soot actually interferes with reduction, no need for soot to get carbon trapping either
  • test, test, test

sorry I took the long video down, need to re do it and make it a bit more interesting. Too many folks were falling asleep at their computers.

Check out Hsin - Chuen Lin out of higher fire pottery. Master potter west coast US, and has a simple method to reduction fire time after time by increasing his gas in his Bailey down draft.

Oops late add: for atmospheric burners, primary air is the air that is directly mixed with the fuel downstream of the burner orifice. Secondary air is all the air that leaks in from any holes, generally the cutout(s) or openings around the burner. There needs to be a place to get additional air to effect a draft, primary air is rarely sufficient to cover this. The secondary air can affect your reduction levels. Turbulence are your friend but up around 2000 degrees the atmosphere is really thin so having enough expanding fuel is super important to keep direct path flow  of secondary air (oxygen) to a minimum so you do not lose the reduction you worked so hard to get.

hopefully some of the above helps

 

Edited by Bill Kielb

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8 hours ago, tomhumf said:

@Bill Kielb Thanks so much, will have to re read that a few times for it all to sink in I think! Lots of good stuff

Take your time, do one thing at a time. Start with a decent Claybody reduction and see if you like the look. Heavy reduction early 1500 degrees for an hour let’s say then medium reduction till the end, simple.

See if you like that result learn how to get your kiln in a neutral balance with your damper say at 1200-1300 degrees then start reducing  at 1500 by raising the gas pressure. If that works for you you can build on variation from a stabile starting point and this becomes more controllable and repeatable. 

Edited by Bill Kielb

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