Help, possible overfiring?

[Sage Real]

Hello, 

we have a propane kiln and have been firing it to a final glaze fire at cone 5. The glazes we are using are Laguna Cone 5/6 glazes. All pieces have survived except the colors we are getting are either completely different than the label or seemingly very muted. I had chalked it up to them being different because it is a reduction firing instead of oxidation but a professor had told me I could just be over/under reducing which is resulting in the color change. I have been following a cone 5 schedule I found online. one of our most impacts glazes has been a burgundy that comes out grey or green instead. This has been very trial and error for me because I don’t know much about this yet, but am looking for any help :).

[Bill Kielb]

some general ideas
If you fire with cones then you will minimize the chance things are severely over or under fired. So first question do you use cones and have you placed them in more than one location to get a sense of how evenly the kiln fires. What cone did this fire to?  The other thing that comes to mind is application thickness and the color of the clay body and it’s similarity to the glaze application examples. Did you apply similar to the manufactures recommendations? And not to forget to mention …….. reduction. Good reduction is an  acquired skill which usually takes some time to master. Reduction usually affects a handful of metals so glazes are usually formulated specifically for the reduction effects and then the firing is done verifying visually whether med / strong, early / late reduction was used for a desired result. So a reduction firing can muddy things up a bit but usually only affects specific characteristics of a glaze. In the beginning It’s rare just to fire to a schedule and succeed as intended at all these things simultaneously. Finally, test tiles - always use test tiles to confirm and dial in your desired look  / firing.

Many of these things require testing, isolating the result, and ………. experience. Seems like you are in the experience stage which unfortunately takes time. Tell us more about how you fire to cone, what is your schedule, maybe post a picture or two of the item(s) fired. How are you reducing and when do you reduce, why, etc…. Do you have successfully fired test tiles?  Folks here can help troubleshoot and hopefully decrease that experience timeline or at least decrease the bad experience timeline.

Edited November 7, 2023 by Bill Kielb

[neilestrick]

Most samples that the glaze manufacturer's show are fired on white clay, and fired in oxidation, unless otherwise noted. Firing them on brown clay will darken them a bit, and firing in reduction will darken them even more.

[Dick White]

17 hours ago, Sage Real said:

Hello, 

[snipped a bunch] one of our most impacts glazes has been a burgundy that comes out grey or green instead. [snipped the rest too]

Your burgundy glaze is likely a chrome-tin red. Grey or green is unavoidable for a chrome-tin in reduction. If you still want a red in reduction, you have to go for a copper red.

[Sage Real]

On 11/7/2023 at 4:45 AM, Bill Kielb said:

So first question do you use cones and have you placed them in more than one location to get a sense of how evenly the kiln fires

Firstly, thank you for your reply I appreciate it. We have been using a cone 5 cone for the glaze fire however we should be using witness cones as well for a more accurate read. So far the cones have folded over pretty evenly on top and bottom possibly a little over? As for glaze application we’ve had a fair mix of under, just right, and over glazes. We’re finding that some of ours are pretty runny. To get into reduction we have pretty much just been closing the flu and waiting to see the orange/red flames peeking out. To be entirely honest I am not confident with how to do it correctly. We are working on test tiles though so we can accurately gauge which colors are most changed through the reduction firing in the future.

[Sage Real]

21 hours ago, Dick White said:

If you still want a red in reduction, you have to go for a copper red

This makes a lot of sense thank you. I’ll definitely look for a copper red and test it out next.

[Bill Kielb]

24 minutes ago, Sage Real said:

This makes a lot of sense thank you. I’ll definitely look for a copper red and test it out next.

In addition, copper red formulations are a nice visual test of how much reduction you have - usually at least medium to get a fairly even pronounced red and whether you are maintaining the atmosphere long enough as copper red will show green / grayish where oxidized. Pretty handy to place throughout the kiln to see how evenly the kiln / firing was. Nicely reduced copper red test tiles and a very nice spyhole flame pictured below for reference.

[neilestrick]

If you don't like how the glazes look in reduction, there's nothing wrong with firing them in oxidation.

[Dick White]

4 hours ago, Sage Real said:

[snipped some]... To get into reduction we have pretty much just been closing the flu and waiting to see the orange/red flames peeking out. To be entirely honest I am not confident with how to do it correctly. [snipped the rest]

To put the kiln into reduction, yes, do it mostly with the damper. If your damper doesn't have measurements on it or beside it, add some measured lines on it with oxide wash. Push the damper all the way in and draw a line on it at the outside edge of the flue. This is your zero point. Then pull it out and draw more lines at 1/4" or 1/2" intervals in towards the flue, labeling them as you go. Now you have repeatable points to set the damper for whatever atmosphere you want to create. In the college Bailey kilns, the damper at 3" - 3 1/2" gives a good neutral atmosphere, and for reduction I am at 2 1/4" to 2 1/2" depending on what else I am trying to achieve with temperature and gas pressure.  Include the damper measurements at each status check in your firing log.