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Hi everyone,

I'm quite new to the world of ceramics. I purchased my kiln in November of last year and have been teaching myself everything  using online resources, the instruction manual of my Paragon Firefly kiln, and have also been getting advice from a friend who has been a full time ceramics artist for several years now.

I mostly make small items - jewellery and miniature dolls. When I started out, I had no problem with crazing. I was using white earthenware slip from scarva (recommended fire temp 1080°C-1140°C), Amaco Velvet Underglazes (recommended fire temp 1040°C - 1220°C) and Scarva GZ2108 Transparent Earthenware Overglaze ( recommended fire temp 1000-1140°C)

The first items I made back in November were Christmas ornaments, which were a reasonable size, about 8cm tall.  I fired the bisque on cone 01, on medium speed with a 45 minute hold. I fired the underglaze on cone 05, on a fast speed, with a 45 minute hold. The overglaze was also fired on the 'fast' setting with a 45 minute hold. I had no problems with crazing, even though I was using three coats of the overglaze. Everything came out fine, and I continued to makes lots of mini pendants and brooches using these settings up until recently.

What happened was that I designed a planter, fired it all on the same settings, and this is when I first experienced crazing. I told my ceramicist friend about what happened, and she suggested that I fired the bisque on cone 04 instead of cone 01. She also recommended that I don't do the hold at the end, as it was unnecessary and a waste of electricity/time. So I made a new planter and bisque fired it on cone 04, followed by the glazes on cone 05. The planter came out fine, so I decided that I would do the same for my next collection of pieces.

Today I removed 6 weeks worth of work from the kiln. Everything looked fine and I has relieved. Then, about 3 hours later, I heard pinging. I was horrified to see the pieces all covered in crazing cracks. I have never, ever had any issues with crazing on my tiny pieces before, not with the firing schedule I used to follow. I decided to fire using the temps my friend recommended me to because I had a planter in this kiln load as well as the tiny pendants.

I wonder if I opened the kiln too soon - I opened it at 90 degrees celcius. In the past I have opened it at around 150 degrees, with no crazing issues whatsoever. That's back when I was firing the bisque to cone 01.

If anyone can advise me as to what I am doing wrong, or point me in the direction of some educational resources to help, I would really really really appreciate it.

I'm going to assume that crazing cannot be fixed be re-firing? I'm sorry if I sound stupid - I am very new to all of this and am trying so hard.

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I don't know if this is also useful information to help figure out the problem, but I noticed when I drilled the holes in the pendants of this batch, the bisque felt softer than usual. I actually had an old pendant to compare it to which was much harder to drill. So is there a chance that at Cone 04/1060 C. the bisque was still too soft? Like I said, in the past I was firing the bisque to cone 01/1137 C.

In the past I have not used orton cones, because there's no way I can see into the kiln. I'm wondering if I should try putting them in for a test firing and see what happens.

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25 minutes ago, neilestrick said:

Cone 01 with a 45 minute hold is not cone 01. The additional heat work from the hold probably pushed it up to cone 1, 2, or 3. Without actual cones on the shelf it's impossible to know what was actually happening.

Please can you elaborate? As I said I am relatively new to all of this. I wasn't having problems on cone 01 with 45 minute hold. I still have pieces which I made in November of last year using those temps, and they don't have any crazing.

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Cones measure heat work, which is temperature over time. Holding temperature has the same effect as firing hotter. In general, a 20 minute hold will get you approximately one cone in added heat work. So a 45 minute hold equals roughly two cones, which would put you at cone 2. The 20 minutes is not exact, though, so it's hard to say just what cone you were firing to. The hold on the 05 glaze firing also means that you're getting to a higher cone, probably more like 03. 

I would either stick with the schedule you know works, or do a firing on that schedule but put cones in the kiln so you can find out just what cone you're actually getting to. You don't have to be able to see the cones during the firing, you can check them when it's cool. Once you know what you're actually getting you can just fire to that cone instead of using the hold. Or like I said, just stick with what you know works, although you're probably wasting electricity with those long holds.

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3 hours ago, SweetheartSister said:

I'm going to assume that crazing cannot be fixed be re-firing?

Next time you do a cone 01 firing of the bisque put one of the glazed pieces in the kiln with a thin wafer of clay under the piece to catch drips, in case the glaze runs, and see how it does. Your glaze might be able to go to the higher cone in which case the pieces could be re-fired. The clay will likely be okay but it's a question of if the glaze can take going that hot without issues.

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2 hours ago, neilestrick said:

Cones measure heat work, which is temperature over time. Holding temperature has the same effect as firing hotter. In general, a 20 minute hold will get you approximately one cone in added heat work. So a 45 minute hold equals roughly two cones, which would put you at cone 2. The 20 minutes is not exact, though, so it's hard to say just what cone you were firing to. The hold on the 05 glaze firing also means that you're getting to a higher cone, probably more like 03. 

I would either stick with the schedule you know works, or do a firing on that schedule but put cones in the kiln so you can find out just what cone you're actually getting to. You don't have to be able to see the cones during the firing, you can check them when it's cool. Once you know what you're actually getting you can just fire to that cone instead of using the hold. Or like I said, just stick with what you know works, although you're probably wasting electricity with those long holds.

Thank you for this, it's extremely informative. The way that I thought the hold worked was to make sure that the temperature stayed  on the desired cone for long enough for it to have an affect, if that makes sense. I didn't realise that it goes up a cone by roughly 20 minutes at a time, which just goes to show how little I know - I will glady admit that I am very uneducated about ceramics and have a lot to learn.

I've been doing a lot of research all day to try and fully understand why this might have happened, and if we can go back to absolute basics for a second, how can we know how to get a good 'fit' with bisque and glazes? The earthenware slip I'm using has a firing range of 1080 - 1140 C, the Amaco Underglazes can be fired between cone 05 and cone 10, and finally the transparent gloss overglaze  I have been using has a firing range of 1000-1140°C .

Given that these ranges cover up to three different cones each, how do I know which cone is best for each firing? From my understanding, the bisque firing must always be a higher temperature than the undeglaze and overglaze firing, is that correct? So how much of a difference would it make if I fired the bisque on cone 01, 02 or 03? Or is it simply a matter of making test pieces, trying out different combinations and seeing which renders the desired result?

I have been reading that crazing can still happen months later if the clay body and glaze are not a good 'fit' for each other. So I suppose I'm trying to work out how I achieve that perfect fit in my firings in the future?

Finally, I think I may have worked out what when wrong with these particular pieces. My guess is that the bisque was not fired high enough, and that when it was removed from the kiln and began absorbing moisture from the air, the clay expanded and put pressure on the glaze. I guess this because crazing has never happened with pieces that I fired on cone 01 in the past. Also, if the firing temp of my clay slip is 1080°C-1140°C, am I right that 1080 is cone 03, and therefore cone 04 would not make the clay hard enough? It seems in general that bisque is fired between 04 and 06, which is why I got very confused. My friend recommended that I fire to cone 04 to avoid crazing, but it seems that with the materials I was using it really didn't work.

Furthermore, in the Q&A section for this overglaze on Scarva, there are lots of people having problems with this glaze crazing on Earthenware. Scarva's response is 'Earthenware glazes are prone to crazing due to not fitting the clay body. Sometimes it is possible to do a high bisque and low glaze but then it can be difficult to get the glaze to fit the clay body. For functional pieces it is much easier to choose a stoneware clay body and stoneware glazes then it is possible to do a normal bisque to 1000oC and high glaze. Hope this helps.'

 

2 hours ago, Min said:

Next time you do a cone 01 firing of the bisque put one of the glazed pieces in the kiln with a thin wafer of clay under the piece to catch drips, in case the glaze runs, and see how it does. Your glaze might be able to go to the higher cone in which case the pieces could be re-fired. The clay will likely be okay but it's a question of if the glaze can take going that hot without issues.

This is a really good idea, I will definitely try this with one of the pieces. The overglaze says it will fire up to cone 01, so it might work. The only problem is that some of the underglaze colours might burn out a bit.

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Crazing drove me nuts when i first started.  i know that dreaded "ping" sound all to well.    With much experimentation i eliminated a few factors.  I started making sure the glaze fit the clay.  My husband is science geek so i learned all about the reason (if you're interested here is what is happening  https://en.wikipedia.org/wiki/Thermal_stress#Thermal_expansion_or_contraction

i use Mid range clay body so i need a mid range Glaze, and had to fire in that range.  i was all over the place before when the crazing occurred due to lack of knowledge on this.  I also discovered not to open the kiln to soon... as hard as that is.  for pieces i spent a lot of time on i go with the side of caution and wait until its fully cooled to room temp.   i've also been checking the specific gravity of my glaze so that its not going on too thick. 

hope these ideas help 

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Glaze fit means the glaze shrinks at a rate close enough to the clay body shrinkage, that it will not crack or shiver off the clay. Just because your glaze and clay are firing to the same cone, does not mean the glaze will fit the clay. Apologies if you know this, but it seemed like you are equating firing temperatures with whether the glaze should fit in your descriptions. 

The root of the problem you are experiencing is you are buying off-the-shelf products. Since you don't control the ingredients you don't have control over whether they play well together in the kiln. Learning glaze chemistry is an option, mixing your own, and adjusting to fit your clay body, but that takes most people a lot of time to work out. If you want to be able to just buy glazes and clay, then you should ask your vendors to recommend clay bodies or families of glazes that should work well. If your glaze vendor's response to using earthenware was don't use earthenware, then you either need to follow their advice and choose a different clay body, or keep your clay body and find a different glaze. 

 

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On 6/28/2019 at 4:41 PM, douglas said:

Glaze fit means the glaze shrinks at a rate close enough to the clay body shrinkage, that it will not crack or shiver off the clay. Just because your glaze and clay are firing to the same cone, does not mean the glaze will fit the clay. Apologies if you know this, but it seemed like you are equating firing temperatures with whether the glaze should fit in your descriptions. 

The root of the problem you are experiencing is you are buying off-the-shelf products. Since you don't control the ingredients you don't have control over whether they play well together in the kiln. Learning glaze chemistry is an option, mixing your own, and adjusting to fit your clay body, but that takes most people a lot of time to work out. If you want to be able to just buy glazes and clay, then you should ask your vendors to recommend clay bodies or families of glazes that should work well. If your glaze vendor's response to using earthenware was don't use earthenware, then you either need to follow their advice and choose a different clay body, or keep your clay body and find a different glaze. 

 

Hi Douglas, thank you for this very informative response. So from what you are saying, I now understand that glaze and body 'fit' being mismatched is what causes crazing, and this can happen even if clay body/glaze are within the same firing range, because it's all down to the chemical composition of both clay and glaze, and if these two are not aligned then crazing or shivering can occur?

I don't feel like I'm at the stage yet where I can try making my own glazes, so this is what I'm thinking I should do to see if the fit is good:

1. Contact manufacturer of the slip I use and find out which clear overglaze they recommend as being the best fit

2. Bisque & overglaze fire test pieces to recommended schedule, using Orton cones to track/record heatwork

3. Stress test by putting in freezer and then in boiling water, repeat. Use ink to check for hairline crazing.

4. If this first attempt fails, repeat until I find the 'sweet spot'.

Something I would like to confirm - you said 'Just because your glaze and clay are firing to the same cone, does not mean the glaze will fit the clay. ' But can crazing also occur if the clay body has been fired to the wrong temperature, even if the glaze and clay body would be a good fit if fired to the correct maturity?

The reason I ask this is that I re-fired the pieces mentioned in my original post to a higher temperature, and the crazing has completely gone. Several days have passed and they are still okay. I believe those pieces crazed because I under-fired the clay body. However, I imagine the thermal shock test is the best way to know for sure if you have a good match with commercial clay bodies/glazes?

It's very important to me that I get to the bottom of this so that I can sell my work with confidence in the future. I'm overflowing with creativity right now, but I need my technical skill and knowhow to be up to scratch! Thanks so much for your help.

 

 

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