1. Home
  2. Pottery Crafts
  3. LONG post on results of "Glazus Interuptus"

LONG post on results of "Glazus Interuptus"

Grab a cup of tea or a glass of beer. This post may be LONG.

I fired the load that I had to turn off after two hours the day before. After reading everyone's posts, I opted NOT to replace any of the cones since they only got to 500F. I won't fail to replace them next time, just in case not doing so is the reason for the following. Here's what happened. The kiln sitter kicked off at just a hair below ^4 according to the pyrometer, even though it had a ^5 cone in it, and the ^5 witness cones had only slightly begun to bend. Thinking over the posts, I thought maybe the cones did have heat memory, or at least maybe the small one did, so I decided it would be best to go by the witness cones and watch the pyrometer. Well, then I couldn't really see the cones very well, so I thought I'd go by the pyro since my husband said it was very accurate last time. I turned off the kiln at ^5, according to the pyrometer. This morning I opened the kiln and all looked well. As I unloaded it I noticed a lot of the pieces have small pinholes. So small you can't see them but they pop like crazy when handled. Some of the pieces, not nearly enough, are perfectly beautiful with no pinholes at all, but some are brown with the same small pinholing. I used three glazes, all by Spectrum, all for ^4-^6, all texture glazes, if that matters. Nothing turned out at all the color on the chart. I used three different clay bodies. The brown is on the same clay body, so I assume that may have been why that color turned out that way, or at least a factor. I was supposed to get a dark green and the other two glazes were muted blue grays. Some turned out cobalt, the others brown and the beautiful, perfect ones are almost a celadon green. The pinholing is on two of the three clay bodies, all of which are ^5 clay, by the way. Oh, and most of the pieces left sooty rings on the kiln shelves. What's THAT about? Mainly, my question here is, why the small pinholing? I assume it is from overfiring because the witness cones were all laid over, including the ^6, the kiln sitter ^5 was almost in two pieces. A couple of the pieces had a small glaze puddle on the shelf. Anyone who will be so kind as to offer their opinions are welcome to do so to my personal email rather than the list, if they prefer. I will probably reply personally so as not to clutter the list. Thank you all, VERY KINDLY for putting up with me! Gaye

Reply to
GaSeku
Loading thread data ...

Hi,

In your original post you didn't mention that you were using cones, or I would have made a comment.

Your glazes will almost certainly be underfired.

Cones measure TOTAL HEAT WORK. Even if you warm the kiln overnight it affects the cones - we had the experience once. Your cones have had two hours extra firing, so they shut the kiln down early. The pieces of glaze on the shelf were probably caused by the glaze falling off because it cracked and didn't stick to the pot. Pinholes are usually caused by under firing - the clay was still emitting some gases through the glaze and/or the glaze didn't get a chance to melt completely.

You can try firing the pieces that are no good again, but the results are usually variable and there's no guarantee that it will be better.

It's a long and painful road!!

Regards

Dave

Reply to
David Coggins

Well, live and learn! Hm, I went by my pyrometer reading which registered ^5+ a little. Do you still think they're underfired?

Reply to
GaSeku

When I reread my reply I saw that it could have been interpreted as rude. I certainly didn't intend it to be rude at all. My question is sincere. Since I went by my pyrometer instead of the cones, do you still think it was underfired? I'll get all this down one day, I HOPE! Gaye

Reply to
GaSeku

For me cones are the only way to ensure my glazes are matured, provided of course that I've got the right cone in :-) Pyrometers only tell you what the temperature is in the kiln at the moment you look at them. They tell you nothing other than that.

Steve Bath UK

In article , GaSeku writes

Reply to
Steve Mills

Gaye,

I agree 100% with Steve. Pyrometers ONLY measure temperature, and usually not very accurately. It is heat work which matures clay and glazes, and cones measure heat work, therefore cones are the ONLY true way to measure maturity of your clay or glaze. (Heat Work = Temperature X Time)

The pyro is only there to tell you how fast the kiln temp is rising, and when you are getting close to maturity. As a rule, bisque firing is less critical, and you can get away with just using a pyro (but still check the pyro with cones occasionally) but for glaze firing, the cone is THE BOSS!!

Regards

Dave

Reply to
David Coggins

Steve and Dave, Ok, I understand that I need to trust the witness cones above any other indicator and will certainly do that from now on. If I were to trust the cones only, then this last load was definitely overfired. That would mean that the two hours it was fired the day before wasn't enough to effect the cones and they read accurately after all. SO, can I refire the pots with pinholes and salvage them or should I cut my losses and move on? Thanks so very much for your help, BOTH OF YOU! Gaye

Reply to
GaSeku

Gaye,

I'm in broad agreement with Dave in that we both are making an educated guess that the pinholes are caused by under-firing. By this token it would be a good idea to fire some new work to the next cone up, and include in the firing one or two pieces from the *pinholed* batch. This may sound extravagant, but I would only put a few pots in the kiln and slow the firing down a bit to compensate for the wide open spaces; it's better than putting a larger amount of work at risk. This would be after all a test firing.

Steve Bath UK

In article , GaSeku writes

Reply to
Steve Mills

Ok, I'll certainly do that, but can you tell me how it can be underfired if it's a ^4-^6 glaze and all of my witness cones are laid over? I used ^4, ^5 & ^6 for witness cones and placed them at various places in the kiln. Sometimes I think I will never get this! Frustrated for sure, Gaye

Reply to
GaSeku

It could be that although you followed the recipe to the letter, some of your ingredients my have differed significantly from the original, depending on who supplied them; this is a common problem. Remember also, if you give 6 Cooks the same cake recipe, they will bake 6 different cakes. Also there is the thorny old problem of Scale accuracy: when doing a test batch a deviation of a very small amount can mean a test coming out with one result and the main batch being completely different; one of the reasons I never do a test quantity smaller than 1 Kilo!

With all this, Pottery is a rewarding, exasperating, exiting, frustrating, but fulfilling way of life, and I wouldn't swap it for anything else.

Keep going, there's lots of support out here!

Steve Bath UK

In article , GaSeku writes

Reply to
Steve Mills

Reply to
Eddie Daughton

Thanks for the encouragement, Steve. Actually, these were commerical glazes. I'm going to do it right this time and make some test pieces from each of the clay bodies I intend to continue using. That should tell me something and may answer all my questions.... about this load anyway! Thank you for being so supportive! Gaye

Reply to
GaSeku

as i understand it you were using cones to gauge and a pyrometer.

as stated by many in here the cones measure the actual heat work. but unlike your clay and the cones the glaze reacts different. I'm guessing that while the cones did lay over they didn't lay over at the correct temperature that you needed to mature the glaze. As many have suggested. refire using un used cones and see what happens

Reply to
MKent41616

Thank you, MKent.

Reply to
GaSeku

InspirePoint website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.