Hairline crack

[ronfire]

I recently installed a Bartlet wall control and now I am having cracks show up.  2 kilns loads with 5 sinks have had hairline cracks from the centre to the rim. When I ran with the sitter and fired I have had breaks but not the hairline cracks.  Bisque to 04 glaze to 6 with close to the same firing time  as with the sitter. First time a  was a med-slow cone 6 with a 100 degree drop and a 30 min hold firing time around 14hrs. Next cone 6   8.5 hr at med and 30 min hold after a 100 f drop. The fan gets shut off after the firing is complete and cools for more than 24 hrs and not opened until 100f. Sinks are in the centre of the kiln. A large bowl of close to the same size in the bottom of the kiln is fine.Could it be the 100f drop and hold  as I have not done that with the sitter.  Most times when I fired with the cones the sinks worked fine . With the controller is should fire better as all the elements work together and not unequal like it does with a  sitter.   Almost ready to give up on the sinks but they sell well.

 

[Min]

If you run your fingernail over the crack can you feel where the glaze is cracked and is it a sharp edge or rounded over? I think the cracks are from cooling too fast through the 1150 - 950 F range. I don't think the 100F drop and hold has anything to do with the cracks.

[ronfire]

Yes the edge is sharp. I have not had this problem before. I could change the program and have the controller cool slower. What rate of cooling would you recommend or is there a schedule available to follow?

Would a rate of 150f down for 600f be a good choice?

 

[Min]

I think my schedule for large or re-fired large pieces  should be okay to try with your sinks. I'ld try just one to start with, put it in the middle of a kiln shelf in the middle of the kiln with shelves with pots both below and above it.

- after the peak (drop and hold for your schedule) then 9999 to 1150F / no hold

- 50 degrees hr / to 950 / no hold

- 9999 degrees / to 480 / no hold

-50 degrees hr / to 380 / kiln off  

I don't know how slow your kiln cools on it's own, if it cools slowly in the 480 - 380F range you might not need that second slow down in cooling to be scheduled in. If it's a thin walled kiln then schedule it in.

[liambesaw]

Dunt!

Forms only glazed on one surface, like sinks, are much more prone to dunting.  And a kiln full of large objects like sinks will cool faster because there's less mass radiating during cooling.  If anything, be sure you're cooling slow through quartz inversion, all kilns cool slowly through cristabolite inversion since it's so low.  Just low and slow, slower the better so there's no temperature gradient.

[ronfire]

Thanks Min I will program that in for large objects to slow the cooling and try that.

Should I use that same formula for cone 04 bisque as well, would make sense to me.

If the kiln cools slower than the program would it send  up a warning or alert or just not power the elements?

I have a Skutt 1027-3 and it usually cools slowly but maybe not slow enough now with the doors open. I do have the vent fan off when the firing is finished but will let it run until the controller shuts down usually at 2100.

The sinks get glazed both sides with the same clear glaze, I have an order for 2 sinks and so far have made 4 with hairline cracks.

The first pair I thought was from a power outage at around 1100f and cooled to maybe around 900f when the power came back.

 

[Sorcery]

21 hours ago, ronfire said:

When I ran with the sitter and fired I have had breaks but not the hairline cracks. 

How different are these 2 instances?

I ask because it doesn't take much to turn a hairline crack into a break.

It makes sense to me, the slightly more accurate control of the controller keeps the temps so, that the hairline crack is just a lesser version of the earlier breaks.

When/How are you cutting your hole?

Sorce

[ronfire]

I start the hole when I throw the sink and then trim it to size when I trim the bottom.

When I did get cracks with the sitter they where large cracks but I thought I solved that by trimming the bottom to be closer thickness with the sides.

 

[Sorcery]

Hole sounds good, I have seen folks cut with an implement when things are too dry, which starts cracks.

I don't know that I have anything for ya, I just don't like when so many energies go into thinking about things that seem to NOT be the problem.

Witness cones?

It all still seems quite random. The old breaks, the new, I feel like they are still related and the different firing just exposes them differently.

Do you dry them right side up?

Sorce

[ronfire]

Because of the size and shape they get dried mostly right side up on hydro-stone bats, then moved to an open rack to finish.  Depends on the drying I will cover them to keep the dryness more even or dry on the rims. 

Maybe I should start a log on drying  just the sinks to see what makes them fail .

 

 

[Babs]

Wonder if crack occurring during bisque. Wipe with a wettish cloth. Sometims minute cracks become evident after bisque if doing this.

Are they "floating" on sand or coils , during glaze firing.

How fast is your kiln dropping on colling?

[Min]

19 hours ago, ronfire said:

Should I use that same formula for cone 04 bisque as well, would make sense to me.

I don't think that would be necessary as bisque can withstand greater temperature variances far better than a matured claybody.

17 minutes ago, Babs said:

Wonder if crack occurring during bisque.

My thought is it probably happened in the glaze cool down as the edges are sharp, if it was a crack that was there prior to glazing the glaze edge would be softened. 

[Sorcery]

On 8/7/2020 at 9:23 AM, ronfire said:

then moved to an open rack to finish.

This worries me for 2 reasons.  

One, the leverage one "slat" can have if pressing up on the bottom of the pot can be enough to break it. How flat is the open rack?

2, the increased rate of drying on the bottom when moved to an open rack.  Further....

If there is a hole on the bottom of a pot, will an S crack be an S? I don't think so, makes more sense it would happen like these cracks.

I'm not familiar with hydrostone, sounds like something recently made legal! Lol...but if your bottom center is staying wet the longest, this is a letterless s crack.

Also, depending when they are being moved to the open rack, I envision a band of wetter between the bottom and top. Which is akward to imagine a piece staying strong with.

I would want it to dry from the top down fully and only. 

What clay? Grog?

The feel of the glaze tells you when something broke, not why, so I don't feel like changing anything of the firing will help quite yet.

I love the look of what I can see of that sink, which is why I'm so excited to "help".

Sorce

[Sorcery]

On a hole note....

I don't throw, I always cut at leather hard, with a thin piece of drain pipe. For planter drainage holes.

I would totally recommend this for you.

Once the hole is cut, I take a round implement, like the round of a lightbulb, and bevel the corner, bottom too. That tends to make the hole rim strong enough to withstand any wanting to crack.

I feel like the crack is starting at the hole. Lol!

 

Sorce

[ronfire]

I like the idea to compress the rim of the hole, might help. I could try and cut the hole with a pipe just after I throw it and compress the edge.

 

 

[Bill Kielb]

2 hours ago, ronfire said:

lI ike the idea to compress the rim of the hole, might help.

I have not seen this mentioned above , you may find it helpful.

Compression  a good idea. When holes are cut in most materials there is a natural peak stress around them. Very symmetric and definitely peaks generally in two spots 180 degrees apart. The solution in many instances is to reinforce the hole or to thicken the section it was cut in enough so it can take the normal stress of drying and firing. Compressing the hole ought to help and if need be, adding a clay washer around the hole during construction and compressing can also make this stronger without a bunch of effort. Besides, no use installing the drain fitting and tightening it down only to here it crack. This area will eventually see additional mechanical stress when the drain assembly is installed. Just an idea that works nicely on large clock faces.

Typical stress diagram of a hole cut in materials below FYI