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

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


"GaSeku" wrote in message
...
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.

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

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

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
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

--
Steve Mills
Bath
UK

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

"Steve Mills" wrote in message
...
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

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

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
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

--
Steve Mills
Bath
UK

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

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
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

--
Steve Mills
Bath
UK