Why Cone 6?

[Bill Kielb]

8 minutes ago, Min said:

Not really getting the parallel here. Yes cone 6 requires zinc or boron (for a well balanced melted glaze) but how you are linking this to Earth's geology melting at cone 10 I'm unclear as to your logic.

Hmm, No worries, if the parallel is unclear my only thought is it is what the geology has lent itself to over many years. Cone six and lesser glazes have generally been cone 10 glazes made to melt earlier. Maybe the better question is how did we end up with cone six? Never found a solid reason for that for sure.

[Min]

@Bill Kielb's post above is split from this thread where the topic strayed into another topic so I split it off for this thread.

Question is "how did we end up with cone six".  

I have a feeling that around the time electric kilns started to become popular with potters the electrical requirements and element wear etc to fire to cone 10 were too detrimental to electric kilns. Elements just wear out too quickly, electrical consumption is higher, wear and tear on the kiln and furniture is greater. This in conjunction with wanting to keep some of the attributes of high fire (vs earthenware) lead to a midrange compromise. To bring the maturity temp of highfire clay/glazes down balanced with the increased cost of clay and glaze fluxes leading to midrange. Perhaps Bristol glazes came into play in bringing the firing range down from highfire also.

Edited September 1, 2022 by Min

[Bill Kielb]

Very probable and definitely included in the popular speculation. Some Bristol glazes melt as low as cone 2 though. 70’s energy crunch? Reduced academic budgets ……… why 6 and not five though? Weird! Never have found a definitive answer.

[Min]

Perhaps it's something to do with Orton Cones? Just throwing out a thought here, chemistry for a ^10 cone is 0.03 moles potassium + 0.07 moles calcium + 1 mole alumina + 10 moles silica. Keep the fluxes as a constant but split the alumina and silica in 1/2 and you get the chem for a ^5 cone. ^5 or ^6, both are midrange. Maybe it's as simple as that? 

[Bill Kielb]

Perhaps. Seems to make a good case for cone 5 though. ….. and It’s the last cone that follows the .5/5 relationship. Cone 4 is .5Al / 4Si. I have never really found a reason though, lots of speculation. Cone 10 on the other hand does make sense as the raw materials tend to lead you there. Probably far different on planet Zircon.

Edited September 1, 2022 by Bill Kielb

[oldlady]

thanks, bill.   i wondered where the beautiful blue zircon earrings came from.  made for my ears.

[Pres]

I had thought that Cone 6 was chosen as it was just over the Earthenware barrier, and for the kilns to heat efficiently that it was the best solution. However, I have seen much in the way of firing and glaze running all the way from earthenware temps to cone 10 electric with lots of recipes working at cone 8. I think the science is there for all sorts of temps, but energy costs do play a part in the puzzle. IMHO, I'll stay with *6.

 

best,

Pres

[Denice]

I read Richard Zakins Cone 5/6 glazes and clay in the early 70's.   I was in college helping load and unload gas kilns,  the professors would be wringing their hands about the gas bill.    Neighbors would call the city and complain about the kilns for call the fire department.   I didn't care for low fire glazes  and didn't see how I could ever own a gas kiln.  Richard's book was a Ta Da!  moment for me.   Cone 5/6 glazes caught on like a wildfire,    basement potters like me loved them.   I like Cone 6 glazes they usually have a gas fired look to them.    You have to work harder to work with C5/6 glazes,  lots of testing  and finding the right clay.    I just ordered a new test kiln that is computerized,   my old one isn't and I want and need  better test results.    Den ice

[neilestrick]

11 hours ago, Bill Kielb said:

Cone six and lesser glazes have generally been cone 10 glazes made to melt earlier.

I think that in the earlier days of cone 5/6 work this was true to some extent, but not any more. The development and marketing of cone 6 glazes prior to 7-8 years ago was very much geared toward making imitations of classic cone 10 glazes. I think this approach was a bad way to do it, though, because it classified cone 6 work as something that was less desirable. It was the poor man's cone 10. It was something you were forced to to if you didn't have access to a gas kiln- the realm of hobby potters, not professionals. However in the last 7-8 years, things have changed considerably. Now cone 6 work very much stands on its own, and is likely more common than cone 10 not just among hobbyists but also professionals.

Plus you can't just drop some boron into a cone 10 glaze and have a cone 6 version. Most all classic cone 10 glazes were fired in reduction, and reducing the melt is only part of the process. Mimicking the look of reduction requires additional changes in chemistry and often makes it impossible to make an equivalent cone 6 version, otherwise we'd have cone 6 shino and tenmoku glazes that are indistinguishable from their cone 10 versions, which is not the case. Since the glaze manufacturers embraced cone 6 glaze chemistry as its own thing and started to explore what could really be done with that chemistry, we have seen some really amazing glazes come to market. The variety of cone 6 glazes now dwarfs what has been been done in cone 10 work.

Personally, when I made the switch from cone 10 to cone 6, I approached it from the standpoint of modifying my tried and true cone 10 glazes. I quickly realized that I was taking the wrong approach. I find cone 6 formulation the be very different than cone 10 formulation, mostly because materials like frits are very powerful compared to most of the standard materials we use at cone 10. It still blows my mind that we can make glossy glazes with 24% EPK in the recipe. Plus the color palette that we can do at cone 6 is huge, and easier since we can use stains that aren't stable at higher temps. I actually find cone 6 formulation to be easier than cone 10.

Why cone 6? No idea. However if we really wanted to conserve energy and extend the life of our kilns, we would be firing at cone 3. We can still make vitrified bodies using feldspar at cone 3, and element life would be fabulous, probably 200 firings instead of 150. If I was working alone and didn't have a community studio, I would develop a cone 3 porcelain and glazes. There's no reason not to. I think we're probably stuck at cone 6 for a good long time, though, because we are at the mercy of the commercial suppliers, and they've got a good thing going at cone 6. I hope that we'll see more schools switch from cone 10 reduction to cone 6, because that is what most of their students are going to be doing when they graduate. They could still use their gas kilns, but cut their firing costs in half.

Sorry for the long post. I spent my first 16 years in pottery working at cone 10, and the last 14 years have been at cone 6, so I have a lot of opinions on the subject. I could write pages and pages about it! Switching to cone 6 electric was the best thing I've done for both my business and the development of my work, but I totally get why cone 10 gas firing is better for some people.

 

[Min]

Looks like it could be cone 6 specifically because of the feldspar required for a white burning stoneware fluxed with feldspar. From this page at Digitalfire:

"Cone 6 is the lowest practical range for feldspar-fluxed white plastic bodies. Vitreous porcelains can be made with up to 30% feldspar (the remainder being silica, 20-25% is needed to prevent crazing, and kaolin/ball clay). White stonewares need about 20-25% feldspar. Typically a bentonite addition is needed to augment plasticity. Buff and brown burning bodies offer more flexibility since they often employ clays that already contain natural quartz and fluxing minerals (so less feldspar powder is needed).

Clays made using feldspar can be made to vitrify to zero-porosity density at cone 6 (including porcelains and stonewares). Since vitreous ware can be made at cone 6, many do not bother with the extra time, trouble and expense of firing to higher temperatures. Companies making prepared glazes serve the middle temperature range with a wide array of products."

and from this page:

"However, making bodies mature a cone lower can be tricky. You cannot just use any cone 6 body and expect to make ware of the same durability at cone 5. White-burning bodies, where the feldspar percentage is already maxed-out to get them to vitrify at cone 6, are most impacted by firing them lower. Any cone 6 body that already has a significant porosity is only going to see it go higher at cone 5. That being said, a fully vitrified cone 6 porcelain easily qualifies as a white stoneware at cone 5. Buff burning bodies high in ball clay can be made to vitrify easily at cone 5 since there is plenty of room for extra feldspar. Brown and red burning bodies will lose color at cone 5. That being said, they are easiest of all to modify for cone 5 (and even lower) since they usually contain red-burning terra cotta clays that both plastify and flux the body, increasing the percentage of these moves the vitrification range downward. Producers that supply photos of the body color at many cones make assessment for cone 5 much easier."

 

[Bill Kielb]

I love everything above. For the most part I have heard many of the same reasons / feedback and also speculate boron helped simplify the melt issue. I am a cone six proponent for sure just on energy use alone.  As far as reduction, I think it’s a great learned skill and an artistic tool. The number six though (cone 6)  - seems pleasantly odd maybe without one central reason ……… but it happened.

Edited September 1, 2022 by Bill Kielb

[Mark C.]

Its never made any sense to me except that cone 06 is not durable at all. In my area cone 6 never made a hold at all until the last 10 years and only one school went to it (including a cone 10 high school thats still cone 10) The school that switched  just did  this year due to  loosing thier tec loosing  funding so the teacher could load and fire and walk away (simplfiy) ands thats our Junior collage. There are a few private folks doing cone 6 now as well. The community art center and our Cal State Poly collage are all cone 10 still.

I now have 50 years in at cone 10 reduction -I would not ever consider oxidation as my customers and I like the reduction look

Edited September 2, 2022 by Mark C.

[Hulk]

On the geology point, I started down that road yesterday, at what temperature(s) does the earth's crust melt?
From there, at what temperatures do rocks melt?

There are many factors and wide range of temperatures, it seems - I didn't pursue the topic very far.

I'm watching the pyrometer on my fully manual kiln, peaking at 2212F, where it's a toss up if the number hits before the sitter trips (fitted with cone 6 small cone).
From there, I'm allowing the temperature reading to drop to 2112F, with middle switch off, bottom on low, top on medium or high, then twiddling the switches to hold there for a half hour or more afore setting all to low for a slow drop to 1850F, then off.

All that will typically: partially drop the cone 5 witness on the top shelf, partially drop cone 6 the middle shelves, and close to full cone 5 on the bottom.

Back to topic, four of the clays I'm using now indicate %absorption at cone 5, the fourth is listed as cone 5/6, and absorption % at cone 6.
I put the 1.2% absorption at cone 5 on the top shelf.

Absorption testing indicates all good.

Edited September 1, 2022 by Hulk
correct the target temps

[Bill Kielb]

2 minutes ago, Hulk said:

On the geology point, I started down that road yesterday, at what temperature(s) does the earth's crust melt?
From there, at what temperatures do rocks melt?

I think on planet Zircon they have been working at cone 20. 

[Callie Beller Diesel]

I think on planet Zircon their fuel costs are lower than ours. 

[Bill Kielb]

4 hours ago, Callie Beller Diesel said:

Zircon their fuel costs are lower than ours. 

I heard they are fully fusion powered now.

[neilestrick]

3 hours ago, Hulk said:

There are many factors and wide range of temperatures, it seems - I didn't pursue the topic very far.

I briefly looked into it as well. Lava has a range from 700C to 1300C, the bottom of the Earth's crust is 1000C, and the core is 5,200C, and minerals melt as all sorts of different ranges, so I'm not sure there is a single temperature that is the magic number for the planet.

[Babs]

 

The author, Janet de Boos, 1978 published info to this extent , referencing the great Ivan England's work in creating  mid fire glazes and their increasing value to the pottery world .

"Glazes for Australian Potters"

ISBN 0 726922129

Some recipes range from C1 to C7 with additions or decreases to make work... mentions the need of working with fickle fluxes Zinc Oxide and Barium and the more difficult path required in formulating glaze recipes at midfire temperatures,  the advantages are considerable however.

Frits 

Edited September 2, 2022 by Babs
Copyright issues

[Kelly in AK]

Min & Bill, thank you for this thread! It’s a rare moment when I can be part of a discussion about ideas that roll around in my head with people who think about the same things. As I learned about clay I thought it was beautiful how these two places (cone 06-04 and cone 10) were natural landing points for firing clay. The bisque range was where most any clay got strong enough to use for everyday purposes. It’s a range beyond an average campfire that’s reachable with a rudimentary kiln. Lead glazes allowed more useful ware by making it sanitary.

The next natural landing point was cone 10. It’s an order of magnitude greater to make a fire burn that hot. Feldspars, abundant in the earth, melt there. The refractory clays (originating from feldspars!) mature there. Pots that were vitreous and much stronger could be produced in this range. 

My conjecture is that it’s all about easily getting to a vitreous strong ceramic. Vitrescence is our modern standard for useful sanitary pottery. Cone 6 is a temperature common materials can be combined into a body that makes vitreous ware. You can certainly tune a body with frit to make mature ceramic at lower temperatures, but frits are far from common materials. There are clays that mature at temperatures lower than cone 6, but they’re all over the place. One will mature at cone 4, another at cone 02. Lower melting feldspars and neph sy, however, which are mined at great scale, melt near cone 6. All of the other factors, such as electric kilns, the desire to conserve energy, boron in a readily available non soluble form, and the economy of having infrastructure (shelves and what not) last longer, created a perfect environment for this new landing spot, cone 6. 

I’m delighted to see how it’s developed over the last two decades. We’ve come a long way from “imitation cone 10.”