bisque fired fine but melted during glazing

[dom92]

I had an odd failure happen yesterday, and I'm not exactly sure what to make of it.  I had used a premixed ^6 porcelain slip to cast a small coffee cup in a plaster mold.  The label on the slip container states:  

Glaze fire at Cone 6 - 2232 F

Bisque fire at Cone 04 - 1940 F

The kiln I used was manual, so I slowly (over about 8 hours) brought the piece up to 1940 F and held it for about 15 minutes, then let it cool on its own in the kiln over the next 12 hours.  Everything seemed successful.  I then applied some Cone 6 glaze and then, over several hours, brought the kiln up to 2232 F.  When I opened up the kiln the next morning I was shocked to find my cup a melted blob on the floor of the kiln.  It was like it turned to lava sometime during glaze firing and just completely collapsed.  What's odd is this failure came at the end of successfully making 3 other cups using the precise same steps and the same slip.  I'm of course hesitant to try to make any others until I figure out what happened here.  I'm thinking of several possibilities:

- My pyrometer is off and I didn't actually get it up to proper bisque temperature

- Bisque temperature was fine but I didn't hold it long enough

- Ramped it up too fast during glaze firing (I have an electronic sitter but haven't hooked it up yet)

- A problem with the slip itself (which I think is unlikely since it's from a well known local manufacturer and since I've had 3 previously successful firing/glazing)

And of course there could be something more simple I'm missing here.  I'm open to any suggestions.  

[Min]

After your bisque fire did it absorb the glaze the same way as your other pots? Any chance you accidentally used a low fire slip for the melted pot?

[dom92]

4 minutes ago, Min said:

After your bisque fire did it absorb the glaze the same way as your other pots? Any chance you accidentally used a low fire slip for the melted pot?

It's hard to say because it melted in to such a messy blob.  The parts that were still identifiable did appear to have glazed just fine.  There's no chance I used the wrong slip in this case.  One other possibility I was just thinking of.  Could this could occur if I didn't stir/mix the liquid slip enough before casting?  

[Min]

Sorry, I meant was the bisque porous when you glazed it.

I can't see it being because the slip wasn't mixed thoroughly. Is there any chance your kiln overfired in the glaze firing? Did you have witness cones in the firing? 

[Chris Campbell]

I think it could be one of two things. 

Your kiln over fired which you should know from the witness cones (hopefully).

Your slip was compromised ... I think if you did not stir it well enough you left the clay on the bottom of the container ... so, the form you made did not have enough strength to get through the high firing. Everything mostly survives bisque ... the test is really the high fire.

[dom92]

13 minutes ago, Min said:

Sorry, I meant was the bisque porous when you glazed it.

I can't see it being because the slip wasn't mixed thoroughly. Is there any chance your kiln overfired in the glaze firing? Did you have witness cones in the firing? 

I didn't have any actual cones in the kiln.  After this I recognize I need to pick some up next time I'm at the ceramics store.  I'm wondering about the kiln overfiring as well, like perhaps the pyrometer malfunctioned.  It didn't seem to be porous after bisque firing.  I put some water in the cup for awhile to check for leaks or seepage and there didn't appear to be any.  

[Min]

17 minutes ago, dom92 said:

It didn't seem to be porous after bisque firing.  I put some water in the cup for awhile to check for leaks or seepage and there didn't appear to be any.  

It should be porous after the bisque firing. Do you have another piece of bisque that you know for sure is from the same batch of slip? If you do take it and touch the tip of your tongue to the pot. Your tongue should very slightly stick to the bisque. How are your measuring what cone you are firing to?

[dom92]

1 hour ago, Min said:

It should be porous after the bisque firing. Do you have another piece of bisque that you know for sure is from the same batch of slip? If you do take it and touch the tip of your tongue to the pot. Your tongue should very slightly stick to the bisque. How are your measuring what cone you are firing to?

Unfortunately all of my other bisque from the same slip has already been glazed.  I'm measuring temperature by using the electronic pyrometer that came with my kiln.  

[neilestrick]

For the kiln to fire hot enough to puddle a cone 6 body, it would have had to go well beyond cone 10. At 10 it would slump a lot, but not puddle. I've never seen a pyrometer go that far out of whack that it could over-fire by 4+ cones, other than putting a type S thermocouple on a type K controller. But that's not happening here. And there's nothing about the bisque that would have affected the glaze firing that way.

Any commercial clay body/slip should still be porous after bisque firing to cone 04, so I'm not sure what's happening there.

Do you have any low-fire slip in your studio? That slip would definitely puddle at cone 6.

Have you checked the program on the kiln to make sure you set it correctly?

I'm not sure you how could compromise a cone 6 porcelain so badly that it would puddle at cone 6. it would take adding a bunch of something in there, like low fire slip or a bunch of frit or talc.

[dom92]

40 minutes ago, neilestrick said:

For the kiln to fire hot enough to puddle a cone 6 body, it would have had to go well beyond cone 10. At 10 it would slump a lot, but not puddle. I've never seen a pyrometer go that far out of whack that it could over-fire by 4+ cones, other than putting a type S thermocouple on a type K controller. But that's not happening here. And there's nothing about the bisque that would have affected the glaze firing that way.

Any commercial clay body/slip should still be porous after bisque firing to cone 04, so I'm not sure what's happening there.

Do you have any low-fire slip in your studio? That slip would definitely puddle at cone 6.

Have you checked the program on the kiln to make sure you set it correctly?

I'm not sure you how could compromise a cone 6 porcelain so badly that it would puddle at cone 6. it would take adding a bunch of something in there, like low fire slip or a bunch of frit or talc.

I have no other slip in my possession right now, so there's no way I could have confused it or taken from the wrong container.  Here's a photo of the actual label:

https://drive.google.com/open?id=1jhW5ELUxn14f38kONTrtvVy80RyG7WB0

I don't have any "program" as my kiln is fully manual with a limitless switch.  I discovered one new potential problem since my original post when I pulled out the manual for my kiln today.  It notes that the pyrometer is not rated for temperatures above 2000 F and that if kiln temperature is raised above 2000 the pyrometer *may* show an error code until temperature drops below 2000.  My pyrometer went up to 2232 F just fine, but now that I know that's above its rated temp I'm wondering if what I was seeing was completely inaccurate?  I'm also wondering if this was as simple as not stirring up the slip well enough before pouring.  Sometimes I'm not as diligent with that as I should be.  

[neilestrick]

So you're just using the pyrometer, no cones, no Kiln Sitter, etc? I'm betting that pyrometer is the problem, then. What brand/model pyrometer is it? Is it digital? What brand/model is the kiln?

[dom92]

2 hours ago, neilestrick said:

So you're just using the pyrometer, no cones, no Kiln Sitter, etc? I'm betting that pyrometer is the problem, then. What brand/model pyrometer is it? Is it digital? What brand/model is the kiln?

Okay, everyone, I think I discovered the obvious problem.  I was following up on neilestrick's last post and looked at the manual again.  I clearly misread things before.  The pyrometer itself has a max temperature of 2372 F, but the thermocouple that came with it is only rated to 1500 F.  I would presume that trying to fire the thermocouple some 700 degrees above its max rating could cause some readout accuracy issues.  I'm an idiot.    I've already got a high temp (2372 F rated) thermocouple on order and it'll be here on Saturday.  I'll hold off subsequent firings until then.  I appreciate everyone's patient replies and advice. 

[neilestrick]

You should have a type K thermocouple.

[dom92]

17 minutes ago, neilestrick said:

You should have a type K thermocouple.

It is

[neilestrick]

24 minutes ago, dom92 said:

It is

Then it should work higher than 1500F. What brand is the pyrometer? Is the thermocouple a really skinny little thing?

[dom92]

4 minutes ago, neilestrick said:

Then it should work higher than 1500F. What brand is the pyrometer? Is the thermocouple a really skinny little thing?

The entire setup is a K2 Instruments model DP-902F.  The thermocouple end is just two wires twisted together with a dab of what looks like solder connecting them.  The K type thermocouple I ordered to replace it was this: 

https://www.amazon.com/gp/product/B00XJ0VUBG/ref=od_aui_detailpages00?ie=UTF8&psc=1

A totally encapsulated thermocouple.  Definitely different than what I have now. 

[neilestrick]

32 minutes ago, dom92 said:

The entire setup is a K2 Instruments model DP-902F.  The thermocouple end is just two wires twisted together with a dab of what looks like solder connecting them.  The K type thermocouple I ordered to replace it was this: 

https://www.amazon.com/gp/product/B00XJ0VUBG/ref=od_aui_detailpages00?ie=UTF8&psc=1

A totally encapsulated thermocouple.  Definitely different than what I have now. 

Ultimately, the thermocouple should only be used to show approximate temperature and rate of climb. You need to be using cones to determine the actual heat work, or you'll never get any consistency in your work, and could do a lot of damage to the kiln.

[Mark C.]

My guess is your clay is compromised as well as your thermocouple

Firing the next pots should be done with cones and a visual on thew ware so you can see if its melting before the cones do.

For clay to melt it has to be hotter than your kiln can go without damage to it.

Thats why I'm saying the body needs more testing as does the thermocouple-cones are the only way to know your temps.

[glazenerd]

Raw clay ( without flux) melts at cone 32 typically. Cone 10 bodies have 25% flux additions and cone 6 typically runs 30%. As Neil pointed out, a cone six body fired to cone ten would slump, but not melt " like lava."  A cone 04 body typically has 40% flux and at cone six would slump hard and melt "like lava" at cone 9-10.  Mark pointed out the slip was compromised: given your description of the mess, I would agree. What concerns me is there only one flux with enough strength to turn clay into a liquid state: cryolite. 

Tom

[Min]

3 hours ago, dom92 said:

The entire setup is a K2 Instruments model DP-902F.

From the info you gave it appears that the pyrometer is only rated to 1999F and the tc at 1742F for a very short time or 1562F for continuous use. 

Simple way to figure out what's going on would be to forget the pyrometer/tc for now and just pick up some cones (5,6 and 7). Mix up your slip really well, skip the glaze and just single fire to cone 6. Like Mark said position the test pot(s) so you can see them and the cones packs thru the same spyhole and fire the kiln manually. (wearing kiln safety glasses)

"Therm oCouple

Maximum operating temperature 1742°F for 15 minutes or less. Maximum continuous operating temperature oF 1562°F. Higher temperatures can be measured, but the therm oCouple life will be reduced at temperatures above 1562°F. Note – the meter is capable oF displaying a maximum temperature oF 1999°F, actual temperatures maybe higher."

[neilestrick]

9 hours ago, Min said:

From the info you gave it appears that the pyrometer is only rated to 1999F and the tc at 1742F for a very short time or 1562F for continuous use. 

Simple way to figure out what's going on would be to forget the pyrometer/tc for now and just pick up some cones (5,6 and 7). Mix up your slip really well, skip the glaze and just single fire to cone 6. Like Mark said position the test pot(s) so you can see them and the cones packs thru the same spyhole and fire the kiln manually. (wearing kiln safety glasses)

"Therm oCouple

Maximum operating temperature 1742°F for 15 minutes or less. Maximum continuous operating temperature oF 1562°F. Higher temperatures can be measured, but the therm oCouple life will be reduced at temperatures above 1562°F. Note – the meter is capable oF displaying a maximum temperature oF 1999°F, actual temperatures maybe higher."

I was looking at that, too, then I found the same model, supposedly a newer version, listed on Amazon that has a rating of 2372. The thermocouple looks like a flimsy piece of junk, though.

Use cones.

[dom92]

10 hours ago, Min said:

From the info you gave it appears that the pyrometer is only rated to 1999F and the tc at 1742F for a very short time or 1562F for continuous use. 

Simple way to figure out what's going on would be to forget the pyrometer/tc for now and just pick up some cones (5,6 and 7). Mix up your slip really well, skip the glaze and just single fire to cone 6. Like Mark said position the test pot(s) so you can see them and the cones packs thru the same spyhole and fire the kiln manually. (wearing kiln safety glasses)

"Therm oCouple

Maximum operating temperature 1742°F for 15 minutes or less. Maximum continuous operating temperature oF 1562°F. Higher temperatures can be measured, but the therm oCouple life will be reduced at temperatures above 1562°F. Note – the meter is capable oF displaying a maximum temperature oF 1999°F, actual temperatures maybe higher."

I'll pick up some cones the next time I'm at the store.  Until now I've hesitated on using them because my Skutt kiln has no spy hole.  The only way I'd have to check a cone would be to open the kiln, and I like to avoid doing that so I don't cause unneeded stress to the piece I'm firing.  

[Chris Campbell]

I still say you should completely stir up the slip and probably sieve it to make sure it is totally blended before using it for slip casting. Your piece could have been lacking enough clay to keep it solid. Anything that settles would be missing and the thing that settles best is clay.

I could be wrong, but for a well cast piece to melt that kiln would have had to run for a longer time and it would have been noticibly HOT on the outside. You would have know that thing was too hot just by observation, no?

[Min]

19 minutes ago, dom92 said:

Until now I've hesitated on using them because my Skutt kiln has no spy hole.  The only way I'd have to check a cone would be to open the kiln, and I like to avoid doing that

Yeah, no, you really don't want to open the kiln to check the cones while it's firing. If it was my kiln I would drill out a spyhole to a size that would take a spyhole plug. Drilling through the kiln jacket would be the hardest part. Stainless will work harden once you start to drill unless you have a very sharp hole saw and drill without stopping until you are through the steel. (might need carbide), Don't go cheap on the hole saw. Pilot hole from the inside going out so you avoid element wires etc. to get your placement. Then go from the outside inwards. To taper the hole through the brick, once you break through the stainless, go to a smaller diameter bit and then taper the hole with a file. (wearing a mask)

[Mark C.]

I just drilled 2 -4 inch holes thru a stainless steel electric kiln jacket. Get a carbide bit hole saw or plan on ruining a new bi metal blade hole saw like I did. Worked well on the first hole was dead before the second hole was thru..

With a new carbide hole saw you will have no issues.

You need a spy plug to see whats going on. You can make the plug from soft brick or buy one.