Hulk

This is a very informative site:
Maturity https://digitalfire.com/glossary/maturity

Hi Miss-T
Looks like Goodwin is still in business; they might be able to help.
Contact Us - Goodwin Electronic
I don't know what the broken part is.
Looks like Potteryworks bought Podmore; they might be able to help:
https://www.potterycrafts.co.uk
 
 

Hi rebs,
By "target cone" I mean the heat-work you intend to complete.
We could just say "peak temperature" - however, not exactly the same as heat-work.
Heat-work is achieved through time and temperature.
Look at a pyrometric cone chart.
Note that the temperature required to bend the cone is higher when the rate is higher.


...where Cone 5 is achieved at 2118 °F when the rate is 27 °F/hour (slow).
At the higher rate, 108 °F/hour (fast!), 2167 °F is required.
I shoot for a 2112 °F reading on my pyrometer, no hold, for that drops a Cone 5 cone reliably.
My kiln doesn't heat particularly fast.
My pyrometer might be a bit off as well; what matters, I believe, is repeatable results - good results, preferably!!
Verifying with cones can be very worthwhile.
From an Orton cone chart:
"Cones do not measure temperature alone. They measure heatwork, the combined effect of time and temperature. The role that heating rates have on the endpoint temperature is observed to be that the temperature required to cause a cone to bend will be higher for faster heating rates and lower for slower rates."
The black clay I've used (Aardvark Clay's Cassius Basaltic* - now "Obsidian") is labelled as a "Cone 5" clay.
I found the clay fired nicely at Cone 5, however, when fired just a bit higher - where the 5 has bent past the 5 o'clock position - the problems compounded!!
I'm using a white stoneware that is advertised as a "Cone 5" clay. It tolerates firing to Cone 6, no problems.
I've used a red stoneware that doesn't tolerate firing over Cone 5½, if that - better at Cone 5.
The red stoneware I'm using now will tolerate Cone 6, no problems.
Some clays can tolerate firing a half cone higher; some clays can tolerate firing even higher yet.
Your black clay may be sensitive to over firing.
Check with cones, keep detailed notes!
If you are depending on the controller+pyrometers only, I would like to suggest that you check the actual heat-work with pyrometric cones.
Set cones at each level, note the heat-work achieved and correlate to the results you see.
* (Formerly Cassius Basaltic) Fired Shrinkage is 14.5% and Water Absorption is .5% when fired to cone 5.

Not to mention when you find honest to goodness clay minerals that are 2 μm and larger! That’s called silt, by the geologists, and it sure ain't  clay. Looks like it. Feels like it. Don’t behave like it though. You gotta squish that stuff between your fingers Ryan! Put it in your mouth to feel the grit. Make a pinch pot out of it and fire it. Then you’ll know for sure what you got.
All hillbilly talk aside, my point is there are no substitutes for field testing.
There was a great deal of data generated after WWII on Alaskan minerals, clay included, as well as detailed analysis of clay in my area (Anchorage) after the 1964 earthquake. That data has been helpful, but didn’t teach me how to differentiate the most useful clay materials from what it’s inter-layered with. It was hands on collecting, testing, and comparing that led me to being able to gather useful clay. A seam of excellent clay may be between layers of silt that, to the eye, are indistinguishable.
Further, my recent post in “Wild clay processing” illustrates another caveat on relying on data alone. It is recorded from more than one source that refractory clay is available from the Sheep Mountain site. Enterprising people in the 1950’s made firebrick from it. I got some delightful clay there, it threw  beautifully without any additions or alteration. No cracking or appreciable shrinkage. Curiously, it didn’t settle when I tried to make terra sig from it. It melted around cone 02. Not refractory at all.  I surmise I was near the source of kaolin, but not near enough. It’s also a place gypsum was mined.
Get your shovel and go have some fun. 

A lot of entry level gardening books on soil health define clay particles solely by size (2 μm and smaller). This is generally fine for defining soil texture, but not useful for defining a pottery clay body since not all particles in this size range have the properties of clay minerals. In addition to regular illite, bentonites, montmorillonites, kaolin, and other aluminum silicates, there may be other particles in this size range that do not add clay plasticity or expansiveness; for example, if I have a bag of pure quartz particles (silicon dioxide) that are 2 micrometers and smaller and then I attempt to add distilled water to it to make a clay body, would the resulting paste still have any plasticity or wet strength to it that would make it useable as a clay body? I feel that I would be lucky if I at least get some kind of melted glass that leaves a mess in a kiln from such a "clay" body. What if I have a bag of dried peat or muck particles that are 2 micrometers or smaller? The resulting paste when water is added would not be suitable for firing in a kiln even if it does have suitable plasticity or wet strength since it would just burn away if it were fired in an oxidizing environment. Similarly, if I attempt to make a clay body of limestone (mostly calcium carbonate) that has been ground down to 2 micrometers or smaller, the resulting paste would just reduce in the kiln, turn into caustic quick lime, and gradually crumble apart as the fired object is explosed to atmospheric moisture.

oh my.  have wanted one of these for some time, and finally got it all together to make one.  16" by 16" by 2".  cannot really believe how fast the water is absorbed and the clay is ready for wedging.  I cover the batch with a cardboard box, and flip the clay when one side gets a little stiff.
hesitated to make one because I could not find the absolute recipe for plaster to water--i mean there were instructions on the bag of plaster, but one needs to be a mathematician or something!!!  had 16 pounds of plaster and finally came up with 5 quarts of water, and it is perfecto.  thought I had better do something before the clay that needed recycling got out of control.
starting to think I will throw with the Nara porcelain going forward, and handbuild with the red and black clay just to keep things simpler going forward.  may need another plaster bat for the white clay.  getting used to the L and L kiln, firing to cone 5 instead of 6, as it seems a bit hot based on the witness cones and the end work.  thank you for all the advice and kind words~~

@Hulk I had just reread that post of yours, and good work with the teapot! It looks like your clay has some grog in it, am I right? It seems like that may play a part in helping prevent drips.
@Min @PeterH I'm thinking of something to be fired on, rather than applied after firing. I wonder if a very stony matte glaze would work, just at the cut edge of the lip. Or brush on a slip that's rougher textured and leave it unglazed, but my Yixing teapot is a very smooth clay body, so that can't be the only thing. 
Is there a way to find out water's hydrophobic reaction to clay/glaze materials, or is that something to just test out on my own?  I seem to recall something about this in the back of my brain somewhere, that you can check this on fired glazes by putting a drop of water on it and seeing how much it spreads out or forms a ball. My clay and glaze let it flow too much, and I wonder if there is a known specific material (ie sodium flux versus calcium, kaolin versus ball clay) that helps with this.  Does sand or grog have an inherent ability to cut the flow? 

Hi rebs,
By "target cone" I mean the heat-work you intend to complete.
We could just say "peak temperature" - however, not exactly the same as heat-work.
Heat-work is achieved through time and temperature.
Look at a pyrometric cone chart.
Note that the temperature required to bend the cone is higher when the rate is higher.


...where Cone 5 is achieved at 2118 °F when the rate is 27 °F/hour (slow).
At the higher rate, 108 °F/hour (fast!), 2167 °F is required.
I shoot for a 2112 °F reading on my pyrometer, no hold, for that drops a Cone 5 cone reliably.
My kiln doesn't heat particularly fast.
My pyrometer might be a bit off as well; what matters, I believe, is repeatable results - good results, preferably!!
Verifying with cones can be very worthwhile.
From an Orton cone chart:
"Cones do not measure temperature alone. They measure heatwork, the combined effect of time and temperature. The role that heating rates have on the endpoint temperature is observed to be that the temperature required to cause a cone to bend will be higher for faster heating rates and lower for slower rates."
The black clay I've used (Aardvark Clay's Cassius Basaltic* - now "Obsidian") is labelled as a "Cone 5" clay.
I found the clay fired nicely at Cone 5, however, when fired just a bit higher - where the 5 has bent past the 5 o'clock position - the problems compounded!!
I'm using a white stoneware that is advertised as a "Cone 5" clay. It tolerates firing to Cone 6, no problems.
I've used a red stoneware that doesn't tolerate firing over Cone 5½, if that - better at Cone 5.
The red stoneware I'm using now will tolerate Cone 6, no problems.
Some clays can tolerate firing a half cone higher; some clays can tolerate firing even higher yet.
Your black clay may be sensitive to over firing.
Check with cones, keep detailed notes!
If you are depending on the controller+pyrometers only, I would like to suggest that you check the actual heat-work with pyrometric cones.
Set cones at each level, note the heat-work achieved and correlate to the results you see.
* (Formerly Cassius Basaltic) Fired Shrinkage is 14.5% and Water Absorption is .5% when fired to cone 5.

Hi rebs,
By "target cone" I mean the heat-work you intend to complete.
We could just say "peak temperature" - however, not exactly the same as heat-work.
Heat-work is achieved through time and temperature.
Look at a pyrometric cone chart.
Note that the temperature required to bend the cone is higher when the rate is higher.


...where Cone 5 is achieved at 2118 °F when the rate is 27 °F/hour (slow).
At the higher rate, 108 °F/hour (fast!), 2167 °F is required.
I shoot for a 2112 °F reading on my pyrometer, no hold, for that drops a Cone 5 cone reliably.
My kiln doesn't heat particularly fast.
My pyrometer might be a bit off as well; what matters, I believe, is repeatable results - good results, preferably!!
Verifying with cones can be very worthwhile.
From an Orton cone chart:
"Cones do not measure temperature alone. They measure heatwork, the combined effect of time and temperature. The role that heating rates have on the endpoint temperature is observed to be that the temperature required to cause a cone to bend will be higher for faster heating rates and lower for slower rates."
The black clay I've used (Aardvark Clay's Cassius Basaltic* - now "Obsidian") is labelled as a "Cone 5" clay.
I found the clay fired nicely at Cone 5, however, when fired just a bit higher - where the 5 has bent past the 5 o'clock position - the problems compounded!!
I'm using a white stoneware that is advertised as a "Cone 5" clay. It tolerates firing to Cone 6, no problems.
I've used a red stoneware that doesn't tolerate firing over Cone 5½, if that - better at Cone 5.
The red stoneware I'm using now will tolerate Cone 6, no problems.
Some clays can tolerate firing a half cone higher; some clays can tolerate firing even higher yet.
Your black clay may be sensitive to over firing.
Check with cones, keep detailed notes!
If you are depending on the controller+pyrometers only, I would like to suggest that you check the actual heat-work with pyrometric cones.
Set cones at each level, note the heat-work achieved and correlate to the results you see.
* (Formerly Cassius Basaltic) Fired Shrinkage is 14.5% and Water Absorption is .5% when fired to cone 5.

Air flow/ventilation (adequate oxygen) almost certainly important!
Hence, powered kiln ventilation, else, leaving the top peep out.
My black clay experience (one bag) indicates slow bisque, thin sections (not thick), and glaze fired just to target cone, and not even a wee bit more, for the fizzing/bubbling ramps up quickly!
Bisque, holding at about 1500F for a half hour or more seemed to make a big difference in eliminating bloat and reducing fizzing/bubbling.
I'll hold on the way up and down, particularly for red and buff clays.
Holding ~100F below glaze fire peak for half hour or so seems to help with the bubbles/fizzing as well, perhaps giving the glaze a chance to heal over
Drop-and-Soak Firing (digitalfire.com)

Air flow/ventilation (adequate oxygen) almost certainly important!
Hence, powered kiln ventilation, else, leaving the top peep out.
My black clay experience (one bag) indicates slow bisque, thin sections (not thick), and glaze fired just to target cone, and not even a wee bit more, for the fizzing/bubbling ramps up quickly!
Bisque, holding at about 1500F for a half hour or more seemed to make a big difference in eliminating bloat and reducing fizzing/bubbling.
I'll hold on the way up and down, particularly for red and buff clays.
Holding ~100F below glaze fire peak for half hour or so seems to help with the bubbles/fizzing as well, perhaps giving the glaze a chance to heal over
Drop-and-Soak Firing (digitalfire.com)

I like recycled clay!
It's a bit of work, on the other hand, the material is "free."
I'm bagging the clay when it is just a bit wetter/softer than what the vendor provides, however, not so wet that it's very sticky - I can wedge it without having to then scrape off my hands, and although some bits stick to the wedging board, it's not much.

Hmm, 1200 is cone 5-1/2 and with a fifteen minute hold probably a solid 6 or 6-1/2 so firing lower definitely something to try. Peak temperature is important in a bisque firing, but how long is super important as well to burn out all organics.  Often  especially for dark clays.

The red clay I use likes a slow, well ventilated bisque. I take it to cone 04, but I think the slow part is more important. The black clays I’ve tried all seem to be even more picky than my red. In my experience, a way to hedge your bets is to also do a slow first part of your glaze fire, similar to the description in Min’s post.

I'm not an artist so functional for me. I do enjoy really pushing  a mug into art"ish" territory though, I make one or two whacky ones every time I make a batch of 30 or so.
I dabble a bit in art, but it's certainly not "fine", lol.
 

Next time I order biz cards, the text on the reverse shall be larger!

Hi folks, sorry I' a day late here. I had to mull over options for the QotW, and didn't know which way to go. However, all is good as I have a question that I don't think has been asked in a while.
QotW: What's your genre, Fine Art, decorative, sculptural, or Functional?
I have played around with all of the above over the years, yearning to express different facets of myself and the clay. There have been early school assignments that would be simple, but could run to the extreme like: Create a closed 6" cube. I have also done other things that started with no reason other than to just build following one small slab on a large slab for a base, no idea where it would go until done and then cutting the base to fit the slab construction. These usually took days while the kids would be doing slab projects. Other times I would have a sketch showing proportions of a slab or slab/thrown project, only to know if it worked when done. However, when it comes to clay I always return to what I love the most, throwing beautiful functional wheel thrown pottery. At the same time though when I go to shows, or look through trade magazines and others I am drawn to the Fine Art, and the sculptural forms often Photographing those that inspire me.
So What's your genre and what do you enjoy looking at?
 
best,
Pres

Because this thread woke up, I read it and was delighted to see a “problem solved” outcome. Happy times.
For others in the future, I want to say @Hulk‘s first response explains a comprehensive and reliable solution to application frustration.
 I’ve never bothered with specific gravity, I’m too lazy, but I generally follow the principles he lays out. My glazes go on smoothly, at the right thickness, and don’t hardpan in the bucket. 
My fast and loose version is this: 
Mix the glaze and sieve it twice. Get enough water in it until it looks right when you dip your dry fingers in it (That’s experience and I believe measuring s.g. will allow you to bypass this “gut feeling” method.). It ought to coat your fingers thinly, not thick or goopy and not running off all watery. It’ll be way too runny and thin to apply at that point. Lots of drips and dries too fast*. Add saturated Epsom salts solution a little at a time until the viscosity is right- stir vigorously, pull out the mixing tool and count seconds. It should stop spinning by four or five seconds. If you over flocculate, add water a little at a time until viscosity is good.
Most of my glazes use frits, not Gerstley borate, which is a unique glaze gelling animal. Less flocculant required for those with much G.b.
*When I say “dries too fast,” I mean the runs and drips where you poured or dipped will have frozen in place before you even finish dipping, pouring, or brushing. I don’t have to wait around for my glaze to dry, but I also don’t have to rush the job to avoid a sloppy looking application. 

Thanks again to Hulk, Min, Callie and others for answering questions I had in 2021... which are similar to the ones I've posted (and you've answered) this year! It's taken so long for some of these concepts to sink in.
Pir

I've gone wetter and jellier.
My post, copied from 
Uneven dipped glaze application - Clay and Glaze Chemistry - Ceramic Arts Daily Community
I'm having better dipping results after employing Tony Hansen's suggestions, here https://digitalfire.com/4sight/library/thixotropy_and_how_to_gel_a_ceramic_glaze_73.html
I'm thinning my glazes (I have some white and light blue premix powders, the rest are mixed from raw ingredients in my studio) to ~1.41 to 1.47 specific gravity (I'm keeping notes on each glaze), then adjusting thixotropy with Epsom salts ...then, much easier to apply glaze in even layer, not too thick, and they "gel" (that is, not run and drip!) better.
When piece is lifted out of the glaze bucket, a good grip helps, for a few sharp shakes helps to lose any drips, then wait for the glaze to mostly gel, and finally, smooth any drips with a wet finger before they set up, where the trick is to not smooth too soon, else the glaze will be thin there, and not too late, for the drip won't smooth. Viola! Hope that helps.
and from
White glaze problems - Studio Operations and Making Work - Ceramic Arts Daily Community
How "wet" the glaze is - ratio of water to solid matter - can be expressed in terms of specific gravity:
Specific gravity (digitalfire.com)
I'm running between 1.39 to 1.47 (per my notes - keep notes!!) for the several glazes I use, mostly mixed from raw materials (I don't plan to buy any more premix, but do plan to use the premix I do have until gone - a powder blue, bad clear, and a matte white). A wetter glaze allows a wider (longer) "window" of dip time to achieve desired thickness, but will take longer to dry. A wetter glaze may also be more prone to running/dripping, however, the undesirable movement can be ameliorated! A less wet glaze typically goes on too thickly.
How the glaze moves can be adjusted:
Thixotropy (digitalfire.com)
This is the hot tip! A wetter glaze that has been adequately "gelled" goes on like a dream! I'm not having to rush to avoid a too thick glaze layer; a shake or two takes care of drops at the rim; any remaining drops I'll just run around the rim; any other remaining drops, I'll wait several seconds for the glaze layer to loose its sheen, then smooth the drop with a wet finger*. Glazing (almost all dip/pour - very little brushing, no spraying) is much more fun for me now.
As I typically don't glaze fire twice a month (if that), I start out with thorough mixing, sieve, then check/adjust specific gravity, leaving thixotropy check for last. Turning the glaze with a large kitchen whisk (clockwise, easier on my hand, elbow, etc.) to the meter of "Positive Vibration" (Bob Marley's 1976 album), when stirring is ceased, I expect:
    the glaze to turn as a mass, with very little shear (iow, not much currents of differing speeds);
    the mass to continue turning for about three to four turns;
    the mass to slow and stop all together - again, very little currents of differing speed;
    the mass to "bounce back" a bit when it stops.
The desired behaviour - liquid when moving, gelled when not moving - is much more conducive to successful glazing than the "liquid until it dries" extreme (imo).
Inadequately gelled glaze will exhibit currents of differing speeds as it turns in the bucket (use round buckets; square buckets are bad, heh) - a portion of the glaze will just keep on spinning for quite some time, whilst shearing against the portion of glaze moving at a slower speed.
*Some prefer to allow droplets to fully dry, after which they can be levelled by "fettling."

I would love to read more about these processes.
However it's worth mentioning that some beds of clays contain:
natural deflocculants (eg humic acids & lignates)
natural precursors to deflocculants (eg lignin) which are activated in alkaline environments (eg sodium carbonate).
So it's quite possible that you could get a usable slip by careful selection of the clay used. (And a poor slip by modern standards would still be usable and open up a craft/industry).
PS Off topic but well worth reading.
A Secret of Chinese Porcelain Manufacture
https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.196306971
The secret of old Chinese porcelain is the technique by which a material, capable of forming extremely thin-walled articles, was obtained from a slightly plastic kaolin. Intercalation compounds, which form by treating kaolinite with decaying urine, give the key to the preparation of this material.
Unfortunately I cannot find a freely available copy at the moment, so from memory ...

It seems that the Chinese eggshell porcelain industry was born when the amazingly plastic  properties of a specific clay bed were discovered.  It was found under the ruins of what had been a long-lived stable, and the properties were due to the action of stale urine on the kaolin particles. They then found out how to duplicate the effect under controlled circumstances.
Basically kaolin occurs as platelets, which aggregate into stacks. Under suitable conditions the large stacks can be delaminated into smaller stacks or even individual platelets -- giving the body the "fines" necessary for plasticity.
 
So in theory:

... and in practice:

My SiC kiln shelves arrived this weekend, I’m so excited… but I accidentally ordered 3/4” instead of 5/8.”
After consoling myself with the fact that they’ll take a lifetime to warp at cone 6, I remembered watching Isaac Button load his kiln.
I feel better now. I encourage anyone with heavy kiln shelves to watch him load his kiln. Around the seven minute mark if you’re short on time. 
 

Hi,
 I finished up welding, painting, and assembling the kiln cart, so we were able to mount and assemble our new kiln's parts on the cart to see if it will work OK.
It seems very stiff. It did not seem to deflect at all. We only need to move the kiln a few feet back and forth from a storage location to an operation location. The floor is smooth and level. I hope I have not made a mistake by building our own cart, but it was fun to fabricate, less than half the price of a ready made version, and made out of the sort of heavy weight materials that are impractical to specify for a consumer grade mass distribution product. 
Thanks for all the help getting started with a new kiln.

 
 
 

 

 

 

 

 

 

I usually put slop out on a piece of backer board, but really wet stuff I hang in a piece of cloth tied up to hang on a hook. weight in the bag, and the cloth just drains to the bucket below.
For wedging I used to use canvas over concrete. Now with dust concerns, I have covered the same with a piece of 3/4 inch plywood. It took a while to get good at wedging on it, but now it works fine and is easy to clean up. The plywood had a coat of kills on it before I started using it. Left over from some renovation project.
 
best,
Pres

I've several pieces of backer board that I use for wedging*, they are still very flat after several years of service.
Backer board, likely not for everyone, as the surface is a bit rough.
I don't mind it; I keep my fingernails away from the surface.
The material doesn't move water like plaster (#1 pottery plaster).
For reclaim, I'm using two inch thick slabs of pottery plaster (cast in a cake pan), which will absorb a lot of water; propped up on 1x1" sticks, five sides are exposed to air for evaporation, which helps a bit as well... 
*one for dark/black clays, one for red clays, one for buff, and one for white, all leftover/scraps from tile projects.