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Accidentally Set Fast Glaze Instead Of Slow Glaze

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Well my week is going just spiffy. Sigh Ran a load of glaze today and it finished in 5 hours 55 minutes rather than the usual 8 hours and 15 minutes (give or take). Checked the settings and realize now I set it for a fast glaze fire not a slow glaze as I usually do. I did at least get the correct cone, a cone 6, so there is that. Max temp on the controller reads 2249, I have a cone in there but won't know what it looks like until I open the kiln tomorrow. This is my first screw up in setting the kiln I guess I just hit the wrong button and didn't pay attention to the read out when confirming the settings.

 

Soooooo have I lost the entire load? What difference does a fast versus a slow glaze fire mean to the pots? Will there be more cracking, crazing, pinholing, will the glazes look different, etc?

 

T

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sorry it happened.  if it had been bisqued, i bet nothing bad happened.  but, can't wait to hear from the firing experts.  only opening it will tell, hope all is well.

 

(maybe you have just proved that you do not need to slow glaze and will start using fast glaze from now on.)

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You don't say what brand of kiln you have, so yours may be different. The manual for my L&L has several pages listing the firing schedules for the various pre-programed profiles. The fast glaze fire profile blasts up to 1949F @ 570F/hr, then drops back to 200/hr until 2199. The slow profile starts off very slowly for a warmup, then at a modest 400/hr to the same 1949, and finishing at 120/hr into the same 2199. In theory, the slow warmup is nice if you have some residual dampness from the glazing process, but it shouldn't make much difference in the outcome, other than time, if you fired at 400/hr or 570/hr up to ~2K. From there, theoretically the faster rate should require a higher end temperature to attain the same cone bend, but the controller cuts them both off at the same temperature. That would seem to result in the fast profile actually reaching a lower cone. Let us know what the cones look like when you open it.

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Thank you old lady for that. I need some hand holding while I wait!

 

I did bisque everything to cone 04, and at least I got the glaze cone setting at cone 6 correct. Just not sure how the glazes are going to react and also not sure why the controller temp went 15 degrees higher than usual. I turned off the vent and am hoping this might help it cool a bit slower and maybe help with pinholes but not really sure if it will make any difference. Nothing I can really do except wait and see when I open it.

 

Been researching slow glaze and fast glaze firing and of course the ones that say no problem I LOVE, and the ones that say I will open a load full of cracked, runny, pinholed, dunted awfulness I hate.

 

Going back to research and obsess some more.

 

T

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How big were the pots?  If they were small to medium, my guess is that there probably isn't any damage.  The glaze qualities may change somewhat from what you expect.

 

Anyway, I think that that you'll be fine.

 

Probably the reason the temp went higher is because of the faster firing-- more heat needed for the same work heat in the end.

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Ray - the biggest were dinner plates. Mostly smaller stuff like cereal bowls, spoon rests, salad plates, etc. was finishing up the firing of a complete dinnerware set, fired half last time. If the glazes are really different might have to redo these so they match. There are a couple special orders in there but they are made up of smaller items as well.

 

Keeping my fingers crossed its down to 420F I ought to be able to open it around lunch or so.

 

T

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In my classroom, I use the "Fast Glaze" setting all the time.  I haven't noticed any difference in the glazes results.  I will note, I do low fire in my classroom though.  

 

That's amazing that it got to temperature so quickly.  Going to Cone 05, my kiln takes just under five hours on a lighter load.  So Just an hour longer for Cone 6 seems really good.

 

I hope everything turns out well.

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I still think you'll be fine.  As Marcia implied, often cooling is much more important to glaze quality than the speed of firing.

 

Many many years ago CM had a piece about a fast fire oil burning kiln which went to Cone !0 in about 3 hours, if I recall correctly, and the potter who wrote the piece seemed pretty happy with his results.

 

Let us know!  The suspense is killing us.

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I opened the kiln and everything survived. Yay... Whew! There were a few of differences that I noticed.

 

The Reds suffered the most, they seemed to run more and were more muted than usual. See attached photo. The top left picture the spoon rest on the left is a previous firing, on the right is this firing. In the stacked spoon rest picture in the top right the top red is a previous firing and the bottom is from this one. The bottom left picture easily shows how much more the red bled than usual. This is the fourth plate of four test plates and it's the only one that the Reds bled into the white like this. They were all glazed at the same time and in the same way. It's kind of hard to see just how different they are in pictures but to the eye there is a dramatic difference with the Reds. Luckily I only had a few pieces with red on them in there.

 

The glazes ran a bit more than usual and blurred some of the edges where two glazes met. The bottom right picture shows my Temmoku bleeding into the white on the right piece the left piece is from a previous firing with a crisper line.

 

Some of the other colors were a bit more muted like the blue, but not nearly as bad as the red.

 

Luckily I had mostly Greens and Browns for that dinnerware set in this load and they seem to be the same as usual. The Temmoku might have a bit more texturing of the colors in it than usual but it's actually kind of nice.

 

Nothing warped or cracked.

 

All in all it was an interesting accidental test. I think I will stick with my slow glaze fire for the most part. If I know I need a load in a hurry and don't have any Reds in it I might consider using fast, but then I might not and just stick to what I know works.

 

Am so glad nothing failed entirely. Now I can do the next step for the custom orders and add the transfers and fire again. But not today the maybe I should take the rest of today off. Lol

 

T

post-22921-0-93581700-1455908492_thumb.jpeg

post-22921-0-93581700-1455908492_thumb.jpeg

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IMPORTANT UPDATE:

 

I just opened the kiln after a transfer firing. I had 3 of the dinner plates that went through the fast glaze fire in this load...

 

ALL 3 CRACKED.

 

I've never had a dinner plate crack like this much less 3 in the same load on 3 different shelves. To have all 3 do it in almost the exact same manner leads me to believe the fault occurred during the fast glaze firing I accidentally did and didn't show up until the transfer firing. I fire my laser transfers to cone 05 on the fast glaze setting. I did do a 2 hour preheat since I needed to get 3 orders of Pug stuff out ASAP. I usually let the transfers dry overnight on the pieces then fire them but I put them in as soon as I finished applying them and set a 2 hour preheat at 200F. All of the smaller stuff came through fine including the matching salad plates to the dinner plates.

 

I have 1 dinner plate left and I am going to do some additional testing on it by putting it my microwave for a few minutes and see what happens and run it through the dishwasher a couple of times then back into the microwave and see what happens.

 

I should also note the only time I have had something crack similar to this was back when I first started and I stacked about 8 6x6 tiles flat on top of each other in a bisque firing. About 3 of them cracked like this. I now stand my tiles upright to fire and it's never happened again. But these plates were not stacked and survived the bisque firing (^04) just fine, survived the fast glaze firing (^6) fine then cracked during the transfer firing (^05).

 

Not really sure as to the explanation of this failure other than a thermal shock fault occurring during that fast glaze fire.

 

I just wanted to add this in here in case someone else has the same question so they know how everything ended up. Needless to say I will NOT be doing a fast glaze firing again.

 

Chalk it up to lesson learned.

 

T

post-22921-0-67800600-1456181180_thumb.jpeg

post-22921-0-67800600-1456181180_thumb.jpeg

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One probable reason for plates cracking during the glaze fire is due to the stresses generated in heating and cooling of the ware.  Large flat objects such as plates and tiles have a disadvantage over bowls and mugs because so much of the object is in direct contact with the kiln shelf.  The ware in a kiln heats faster than the shelves and cools faster than the shelves. This situation results in an additional expansion / contraction  stress than just the temperature change.  The plate has to slide across the surface of the shelf. The shelf contact area also  effectively increases the amount of material to be heated or cooled and thus extends the time for the object to "even out" its temperature.  One way to mitigate this is to reduce the contact area with the kiln shelf.  

 

On objects that have large contact surface with the shelf,  like plates, tiles, etc., I always place wads of clay between the shelf and the ware.  For a plate I will prepare 6 or 8 thumb sized balls of clay and attach them to the bottom of the plate with Elmer's white glue when I put the plate in the kiln.  This keeps the ware off the shelf by about an half inch or so.  The plate then heats and cools more uniformly and can shrink/expand easier. The glue burns out and the area of contact is not enough friction to keep the plate from moving if it needs to. 

LT

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LT - I should have clarified. I place my large flat pieces on bisque cookies. These were on cookies. Since I haven't had this issue with plates before and these plates went through that fast glaze fire setting I did by accident I am going with the presumption that did something to them. Too fast a heating causing invisible fractures or weakness in the clay so when I fired them for the 3rd time to a much lower cone for the transfers these areas failed.

 

Just wanted to update anyone doing a search on a like issue that all did not end well.

 

T

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