Cracks When Refiring

[Celia UK]

Having just installed a digital controller to overcome glaze cracking I have had more success with smooth white earthenware bisqued to 1000oC and glazed with transparent glaze to 1100oC. However, after a wax resist 'accident' in my studio (don't ask!) I had 2 pieces which had clearly been splashed with wax and the glaze had not taken in small areas. Apart from this, the rest of the glaze was fine. I touched up the bare spots and refired in the next firing. It was ramped 100o per hr to 300oC then full to 1080oC with a 15 minute soak. Both pieces now have a long crack spiralling around them. Other than the wax resisted spots they were fine after the first firing (to 1100oC). The other pieces with the same glaze, in the firing were fine. I don't know why this would happen with a second firing, to a slightly lower temperature. Any ideas?

Not sure if the pics show the problem clearly.

[Guest JBaymore]

Having just installed a digital controller to overcome glaze cracking ........

 

Can you please elaborate on this comment?  Are you talking abouty "crazing" (the tiny fracture lines in a glaze that does not fit the underlying clay body)?

 

best,

 

..............john

[Celia UK]

John - The cracks I have always been talking about aren't crazing, they have always been linear cracks, like the ones in the photos. You have both been very helpful to me previously and I also had the kiln engineer in to check my kiln. It was after borrowing a digital controller and having successful firings that my husband bought me a controller.

 

The first couple of firings (bisque and glaze) with the borrowed controller were 100o per hour to 600o C then FULL to 1080 o C. This was suggested by the kiln engineer and made sense Norm with the info you gave me about quartz expansion at 570 oC (I also read this in another document around the same time, so things were beginning to fall into place!).

 

I've realised as I'm reading your posts, that somewhere along the line I've misread something as I then programmed the new controller for 100 oC an hour to 600 oC for bisque BUT 100 oC an hour to 300 oC for the glaze program (I would have sworn I read this somewhere recently but looking now at the only document it could have been, all the 1st ramps in various programs, are to 600 oC! I really must be losing the plot somehow!) This would leave me to believe then, that quartz expansion / contraction was the problem and my cracks are glaze cracks, does that sound right to you? But can you tell me - as I'm now getting into all this and am still curious, why the pieces did not crack in the first glaze firing, which followed the same program?

 

I would add that I have had no problems at all with some pre-mixed brush-on glazes,which seem to be much more tolerant!

 

OK - so I like the document you gave the link to Norm - I will print that off and do the maths to give me the temperatures in Celsius and

I will re-program the controller.

 

NOW to slowing down the cooling. As I thought I'd sorted the problem with more controlled ramping, I didn't included this factor when setting up the new controller. But, especially now the kiln is back in the garage (needed it out of the way for cooking Christmas lunch!) - which has a much colder ambient temperature, I'd guess cooling down quickly will be more of an issue, than kiln in kitchen. Given the additional info I've been able to provide you, and my promise to re-program according to your very helpful document, do you still think I need to slow down the cooling rate? If so, can you suggest the rate(s) for firing down ramp(s)? Do I do this after a soak at the top temperature?

 

I really can't thank you enough - I feel like the 'slow' child in the class who needs everything to be explained at least 3 times, slowly, and in different ways, before it makes sense. (Not good for a recently retired Primary (Elementary) school headteacher (principal)!!!) After the most recent incident (Christmas morning kiln opening, photos above!) I was beginning to wonder if it was all worth it. Once I've got the firing issues sorted, I'll be banging on about glazes and glazing - my next intellectual challenge!

 

Happy Christmas to you and yours and thanks yet again ('til the next time!) Celia

[Mark C.]

I think this is a Cooling crack-your kiln is cooling to fast

You can ramp it down or

Fill it tighter with more work or

Insulate it better or some combination of these

 

Cooling is just as important as heating with some glazes/clay bodies

Mark

[Babs]

You are firing in you rkitchen?? Do you have a good venting system/

[Guest JBaymore]

In the one picture the nature of the crack is ALMOST legible.  I'd need a slightly larger close up of the crack and the way the glaze is behaving right on it.  It LOOKs from the current photo that the glaze is not fractured through sharply, but is filling IN the crack.  This would indicate that the crack happened before the glaze was fully fused and then cooled enough to set.  The glaze flowed into the crack.  That indicates the HEATING cycle if that is true.

 

If the crack in the clay body is forming on the cooling cycle, then the overlying glaze will already have melted.... and started to set... and the fracture in the glaze at the point of the body crack will be sharply sheared through.  Like a cracked window pane. 

 

If you can get a close up of the crack in clear focus...... that would help.

 

If a clay body is not well formulated... it can be fine in a single shorter firing.  But if you extend the length of the firing or refire it... it can have the development of the cristobolite form of any non-melted free silica in the body.  This form of silica has some SERIOUS expansion and contraction issues (see Norm's comments above).  This can make the body prone to cracking (dunting) on uneven cooling.

 

OR... if there is a lot of cristobolite present in the body to start with,....... it can crack on the heating cycle.

 

Even lacking cristobolite....... standard alpha-beta quartz inversion on heating and cooling can cause cracks if the pieces heat unevenly, are heated too fast, or are cooled too fast. 

 

Also exacerbating this issue is serious thickness variations in the cross section of the piece.  If the rim is thin and the base is thick....... the rim heats and cools faster than the base.  This sets up stresses in the piece and cause it to crack due to uneven expansion/contaction.  The type of cracks I see there may boint to this characteristic.  (Is the bottom of the form thicketr than the rim?)

 

best,

 

..................john

[Celia UK]

Thanks all - off for Boxing Day family celebrations now, so will read all your comments carefully later. Just to let you know I wasn't totally off my head with the 100 oC up to 300 oC then FULL - glaze program. I found the document where I'd seen this, which came up from CLAYMAN supplies, (UK) when I Googled 'firing programs'. It was detailed as the glaze schedule for earthenware - which is the clay I'm using. Given all the other info you have provided, I really don't know where this fits in and will be binning the aforesaid piece of paper immediately!

Have a good day. Celia

[Mark C.]

Refiring is a crap shoot at best with some bodies and some glazes

Its never asure thing second time around.

Mark

[Celia UK]

Babs - venting system is a window open at each end of the kitchen creating a through pull of air.

 

John - the cracks are definitely sharp like a crack in a pane of glass. (A few weeks ago, it was a 'crazing' problem, but this certainly does seem to have been resolved by the use of a controller - slower heating cycle than when I was doing it manually. Each problem seemed to be superseded by the next, but I THINK I'm making progress thanks to the support of the CAD forum.)

 

Mark/John - I too am increasingly convinced this is a cooling issue. John - I would normally have the kiln as full as possible, but I needed a piece for a Christmas present and didn't have a compete load on this occasion. I will however bear this in mind for the future.

 

Norm can you confirm for me if the 2 critical temperatures - 570o and 220o are important both on the heating and cooling part of firings. And how about the bisque firing - what is the significance here?

 

Norm - I do appreciate what you suggest about adjusting the glaze - but I'm just not ready to mix my glazes from scratch (yet!).

 

Thanks again all - a few questions remaining in my last 2 posts, Celia

[Bob Coyle]

 I have to agree, it does look like a thermal stress crack caused by uneven expansion/shrinkage. When I do a shutdown from cone 6 I get a drop of better than 1000 deg F/hr for the first couple of hundred degrees. Since I am using a groggy clay, I have had no problems, but if you just did a shutdown after you reached temperature, you may have produced enough of a temperature gradient between the base and the lip to cause the type of crack you posted.

 

Here is a more graphic presentation of the power point Norm attached

KilnFiringChart.pdf

KilnFiringChart.pdf

[Chilly]

 

May I suggest reading this excellent Power Point Kiln Firing Guide "Lecture #8" from Alfred University.

http://claystore.alfred.edu/rawmats/presentations/Raw%20Mats/Class-8%20%28firing%29.pdf

 

Thanks Norm for posting that, has helped me get my mind round inversion.

 

The chart on page 33.  I'm assuming it is using degrees C - for the rate/hr column?.  Looking at my charts of Orton cones, they use a ramp rate of 150C, which is same/similar to the chart.    Now all I need is to see the slow cooling written in the same way, and I might have cracked it. (Cracked the firing that is, not the pots.  ha ha.)

[Celia UK]

We can wait for Norm to wake up Chilly, but I suspect the rates are oF - I've come across some very slow firing schedules in my reading around this.

 

I agree with you - this presentation is very helpful to me in my understanding of the whole process but I also feel I now need a cooling program written in a way that I can put into my controller. CAN ANYONE HELP HERE?

 

Thanks Bob for the chart - thus one already lives close to my kiln - but certainly helps to look at it alongside Norm's presentation.

[Guest JBaymore]

Celia,

 

Don;t think of the firing cycle as ending at the highest temperature. Just keep putting in segments that got up AND DOWN until you let it cool off naturally.

 

best,

 

......................john

[Chilly]

Deleted original text while I work out how to show tabular info in this box.  Looked fine before I "posted".  Be back soon..........

[Celia UK]

Thanks John & Norm - nearly got it clear now.

 

Can you confirm or otherwise - what rate would be appropriate for firing down if I'm only firing to cone 03 max - is the Mastering Cone 6 Glazes info equally applicable? If I were being cautious and heating steadily all the way down to 200oC, is 100 oC / hour slow enough? Alternatively could it be slow to 800 oC, quicker between 800 oC and 250 oC, then slow again through to 200 oC? i.e. Slow at the 2 critical points? I am not aiming for any special effects - currently using one transparent glaze and one white satin, other than a few bottled low fire Amaco glazes from time to time.

 

I haven't recorded the rate my kiln slows naturally - have observed that the temp falls rapidly at the top end and slows down as it gets lower. I would guess that 100oC an hour might be what it does at the top end if just switched off, (but it could be faster than this) and I'd say it's probably slower than this when it gets down to the last 300 oC. But that's just a guess. I would need to know that whatever rate I set is in fact slower than 'normal' as that's what we're aiming for.

 

Thanks in anticipation - again! Celia

[bciskepottery]

The Jan/Feb 2014 issue of Pottery Making Illustrated has an article by Deanna Ranlett on down-firing; pages 13-14, Up and Down.  She posts two down-firing schedules, one for low fire bisque (cone 04) and one for medium fire glaze (cone 6).  Deanna owns Atlanta Clay/Mudfire Clayworks and Gallery.

 

Cone 04 Bisque Firing.  The low fire bisque is for down-firing and slow cooling of large sculptural ware:

 

50 degrees F per hour to 150 degrees F; hold 2 to 6 hours depending on size of work and its dryness

150 degrees F per hour to 200 degrees F; hold for 15 minutes

250 degrees F per hour to 1000 degrees F, no hold

180 degrees F per hour to 1150 degrees F, hold 15 minutes

300 degrees F per hour to 1800 degrees F, no hold

108 degrees F per hour to 1900 degrees F, hold 5 minutes

On the way down:

150 degrees F per hour to 1500 degrees F, no hold

Cool naturally from 1500 degrees F.

 

For large scale work, she recommends cooling through quartz inversion (1063 degrees F) at a rate of 200 degrees F per hour, then allowing the kiln to cool naturally.

 

Cone 6 Glaze Firing

 

100 degrees F per hour to 220 degrees F, no hold

350 degrees F per hour to 2000 degrees F, no hold

150 degrees F per hour to 2185 degrees F, hold 15 minutes

On the way down:

500 degrees F per hour to 1900 degrees F, no hold

125 to 175 degrees F per hour to 1450 degrees F, no hold

Cool naturally from 1450 degrees F

 

I'm sure you can adopt these to your firing range of cone 03. 

[Babs]

How disappointing for you Celia, but pottery is a continuous learning curve, boredom will never exist again!

My old kiln takes forever to cool, Aussie temps help this, but I guess with new light bricks and ceramic board the firing down is imperative not only for glaze effects..... I am purchasing a new kiln and I am reading all of the above very carefully.

I think you had better go to the topic ' the most frustrating thing about clay is.."

In My ceramic fibre gas kiln, now dead, my platters at the top of the kiln, top loader, often cracked in winter, which was a beech as that's a great time to throw platters in a hot clime.

[Celia UK]

Really helpful Norm - thank you again! I think that's the final piece if the jigsaw I need. I've been making notes - it's like being back at college. Now to pull it all together and come up with a programming cycle that covers all the bases and I should be there! Easy peasy.......NOT!

 

Looking forward to a creative & successful 2014 - thanks to all the advice given so generously!

 

Celia

[Chilly]

I'm going to start a new thread, I've spent an hour trying to convert the deg F ramp rates to deg C and failed.