Porcelain

[Robert spicer]

I fired my first pots thrown with P-5 porcelain and was very disappointed with the results. I had plates warp and bowls go out of round. Having never fired porcelain before I can only guess the kiln was fired too hot. Any thoughts

[Norm Stuart]

Over-firing is a good guess. Posting some photos would be useful.

Porcelain fired hotter than its vitrification temperature makes it overly plastic and has a distinctive look as gravity deforms the shape. Fired hotter still, porcelain ware it becomes a pancake. 

Ware shape can help porcelain resist warping, as Digitalfire explains:

http://digitalfire.com/4sight/glossary/glossary_warping.html

What are the two key causes of firing warpage in porcelain?

Here is an example of how a profile having no inherent strength can warp during firing (the one on the left is just bisque fired, the one on the right is fired beyond zero porosity to achieve translucency). Two key factors contribute to this failure: This porcelain is highly vitreous. This shape is vulnerable to warping. If the lip were flared out, for example, it would have much more strength to stay round. If the porcelain was less vitreous it would warp less. Of the two factors, which contributes more to the warping for this specific piece? The shape.

Ware trimmed to uneven thickness, or which accumulate stress from uneven drying are revealed in the kiln as deformations which tend to look more like they're responding to forces other than gravity.

[Joseph Fireborn]

P5 warps very easy if fired over cone 6 even slightly. When I was using P5 if my self supporting cone tips were touching the shelf the P5 would warp. If they were perfectly bent to cone 6, meaning the tip is bent to the base, then they wouldn't be warped. A cone tip touching the shelf or almost touching it is only a few degrees higher than cone 6. 

I eventually quit working with P5 because I disliked it because of this reason. The clay body throws fantastic and drys very well for a porcelain, but the warping was so annoying. I also had a lot of glaze fit issues as well on that clay. I much prefer Frost by Laguna, although it has its problems as well, but it is much more resilient to warping and glazes fit a lot better, or at least the ones I use do.

[Norm Stuart]

Adding boron to a porcelain clay body both:

lowers the cone and;

expands the temperature range of structural stability after vitrification.

I love the whiteness of the various incarnations of Laguna porcelain made with New Zealand Frost, but the COE is quite high at 6.99 making glaze fit a problem.

http://www.lagunaclay.com/clays/western/wc437.php

I've experimented with dipping bisqued Laguna Frost ware in a solution of boric acid before firing to achieve more translucency, which is interesting. Making a clay body from scratch, a high-boron frit like Ferro 3134 works out better.  Porcelain is kaolin, feldspar and lower temperature flux like nepheline syenite or frit, all plasticized with 4% or so Veegum white burning bentonite.

Laguna's Hagi Porcelain, although not always as white, has a more typical COE of 5.6 which fits most glazes as well as their stoneware. Hagi Porcelain is also 30% less costly than Frost.

http://www.lagunaclay.com/clays/western/wc861.php

The product range of Laguna clays in their Orlando, Florida hub is far more limited than here in Los Angeles.

Orlando is even more limited than Laguna's Byesville, Ohio location, which is a practical limitation with transportation costs. In Los Angeles Laguna delivers 1,000 pounds of clay, or more, for $35.

http://www.lagunaclay.com/clays/southeastern/cone5-6.php

http://www.lagunaclay.com/clays/western/cone5-6.php

http://www.lagunaclay.com/clays/northeastern/cone5-6.php

[Robert spicer]

Joseph Rosenblatt said:

P5 warps very easy if fired over cone 6 even slightly. When I was using P5 if my self supporting cone tips were touching the shelf the P5 would warp. If they were perfectly bent to cone 6, meaning the tip is bent to the base, then they wouldn't be warped. A cone tip touching the shelf or almost touching it is only a few degrees higher than cone 6. 

I eventually quit working with P5 because I disliked it because of this reason. The clay body throws fantastic and drys very well for a porcelain, but the warping was so annoying. I also had a lot of glaze fit issues as well on that clay. I much prefer Frost by Laguna, although it has its problems as well, but it is much more resilient to warping and glazes fit a lot better, or at least the ones I use do.

You are right, the P5 throws beautifully. I really enjoyed working with it on the wheel. Would firing to cone 5 be better for this clay body and produce less warping.

[Joseph Fireborn]

It would help, but it would also increase the absorption rate. The best thing would to be to find a nice place in the middle of Cone 5 and Cone 6. Something like 5.5 or as close to 6 as you can get without going over. It would take some testing.

Remember what Norm said, certain forms are always going to warp more in porcelain than in stoneware. It is just the nature of the beast. You can increase the strength of pots by adding structure to them. Like a rim, or a rolled rim, or say pinching 3 corners into the rim. etc etc, anything that adds structure to the pot will help to prevent warping.

[Robert spicer]

Would their be a downside to using a cone 10 porcelain and firing it to cone 6

[Joseph Fireborn]

Yes, you would have to find glazes that fit cone 10 clay, but melt right at cone 6. This would be very difficult.

Read: http://digitalfire.com/4sight/education/the_effect_of_glaze_fit_on_...

and: http://digitalfire.com/4sight/education/understanding_thermal_expan...

and: http://digitalfire.com/4sight/glossary/glossary_glaze_fit.html

Also, if your making anything functional, it won't be vitrified. If your going to move up to avoid warping, you would be best to go to something that has a cone 7 top end. Something like 5-7 range. 

What type of kiln do you have? Do you have a digital controller? If so you should be able to fire to cone 6 pretty easy with a good series of test with pyrometric cones.

[Norm Stuart]

If you're willing to eventually invest the money, you'll never regret replacing witness cones and a kiln-sitter with a digital controller.

http://www.clay-king.com/kilns/kiln_parts/kiln_controllers.html

Before we began down-firing with a six-hour slow-cool, I had reprogrammed the pre-programed ^5 Cone Fire program to fire to a ^5.5 using the "Cone Offset" option to add 32 degrees F to the target temperature.

The controller actually fires accurate cones with an accumulation of area under the curve, recording the temperature every few seconds. Digital controllers make firing a kiln no harder (or easier) than using a microwave oven.

When we made the change I initially validated firings with several witness cones at ^016, ^06, ^04, ^5 and ^6 to discover any quirks, then again as the kiln aged. With perfect cones every time, you can focus on the other variables - of which there are many, and frankly you lose interest in repeatedly identical witness cones.

Digital controllers also add easy down-firing (slow-cooling), and candling (pre-heating) for 0 to 6 hours or more and costs virtually nothing yet can eliminate a lot of problems.

We have a Cress modified Bartlett V6-CF which sells for $430 and directly replaces a Kiln Sitter. We've been very happy with it over the past five years - and our kiln is outdoors with merely a stainless steel sheet screwed onto to the lid to protect the electronics.

The Clay King website sells the Olympic versions of the Bartlett controllers, down at the bottom of the webpage.

http://www.clay-king.com/kilns/olympic_kilns/electro_sitter.html

When the $35 nickel-chrome Type-K thermocouple needed replacement after a few hundred firings, we replaced it with a $125 platinum Type-S thermocouple. The Type-K loses accuracy as it ages and has a much shorter life-span but, from my n=1 experience, not so much that you notice a difference in witness cones. The change-over required switching the Thermocouple Type in a hidden menu, about ten buttons to push in total.

[Joseph Fireborn]

What Norm said is how I feel about it all. I have never fired an older kiln without a digital controller. I have only been doing this for 1.5 years, but I spend 5-6 hours a day doing this stuff, reading, advancing, updating everything. 

When I was over firing I did what Norm did. I added a cone offset. Eventually I ended up using my own temps, then I built my own schedule, which I have tweaked and continue tweaking every firing. I also slow cool, not to form crystals, but to keep my glazes looking the same no matter how full or empty the load is. My current schedule is 14.5 hours for a cone 6 glaze firing and slow cool.

My glazes look fantastic though, and I can advance and improve them with each firing because I have removed the randomness of cooling depending on load volume.

If you're serious about firing electric, get a digital controller. I mean it can be done without it, I know many people who don't have digital kilns, but you're going to have to worry about things like, not over firing, however when you have a digital controller and TC's you just fire, your worries are about glazes, form, improvement and testing. Not the kiln, which is how it should be when your doing electric.

[Norm Stuart]

As Joseph said, firing a Cone 6 porcelain clay to Cone 5 leaves it not fully vitrified, so eliminates the warpage. 

Firing Cone 10 porcelain to Cone 5 isn't even slightly densified or vitrified. Better to simply use a white Cone 5 Stoneware. The concept of porcelain is it densifies so fully it essentially becomes glass, so firing to exactly the right cone is essential as molten glass is very plastic.

Although the COE of Cone 10 clays fall in the same range as Cone 6 clays, we experienced a unique glaze problem firing Cone 10 white clays to Cone 6. The clay remains so porous at Cone 6 that many glazes run through the body of the clay like a sponge, leaving a pool of glaze at the base and pin-holing on the rest of the ware. Not at all terrific.

As a result, we sometimes purchase high-iron Cone 10 clays like Amador which densify better at Cone 10, but the Cone 10 White clay I purchase is Laguna's Max's Paper Clay.

Laguna's Orlando branch sells Sybil's Cone 6 paper clay, which I'd prefer to buy, but this would involve shipping cost from Florida. So I simply dip bisqued paper clay sculptures in a solution of potassium carbonate and lithium carbonate and leave them to dry before glazing to achieve a Cone 6 densification with a Cone 10 paper clay. This adds the extra flux the Cone 10 paper clay is missing to densify at Cone 6.

[Joseph Fireborn]

Although the COE of Cone 10 clays fall in the same range as Cone 6 clays, we experienced a unique problem firing Cone 10 white clays to Cone 6. The clay remains so porous at Cone 6 that many glazes run through the body of the clay like a sponge, leaving a pool of glaze at the base and pin-holing on the rest of the ware. Not terrific.

Learn something every day. Thanks Norm. I didn't understand this accurately. 

I think your best result is going to be to find a nice white body stoneware, find a ^5-7 porcelain, or fire to ^5.5-6. Which of those you choose is up to you. I would pick which ever is the best for you. If your like the p5, learn to fire cooler but still very close to cone 6. There isn't huge functional absorbtion rate difference in p5 between ^4 and ^6. At ^4 its 3.3, and at ^6 its <1.

So if your firing at like ^5 your going to be somewhere in between, which is plenty good for a functional pot. There are a lot of stonewares that are marked as cone 6 but have absorption rates of around 2.5-2.75.  

If your firing p5 to cone 5-5.5 I think you will be absolutely fine. Ideally you want the least absorption possible, which is why I was using p5. I wanted that <1 absorption rate so that I could use my pots in the microwave.

If you start getting above 5% absorption rates I wouldn't label them as microwave safe because they are gonna get hot quick over the lifetime of their use.

I work with Red Rock which is also <1 at cone 6 for this very reason.

Good Luck.

[Norm Stuart]

Joseph - It's true that the the COE of commercial clays tend to increase as the maturity cone declines. This is an artifact of cost control as sodium and potassium are the least costly way to reduce the cone maturity.

Using lithium as a flux, you can make extremely low COE Cone 6 "flameware clay" - far lower than the COE of any Cone 10 clay. Flameware is a clay body which can be heated over the burner on a stove because the difference in expansion between the hot parts of the ware and the cold part is not enough to fracture to pot.  Flameware requires an engobe as most glaze can't withstand thermal shock.

Boron, a flux in Cone 6 and Cone 06 glaze, does not increase COE and also helps glaze and clay bodies with a COE mismatch fit properly.  Ceramic research papers show that boron allows the glaze to have one structure where it meets the clay body and a different structure further from the clay body.  This could be a reason Cone 6 and Cone 06 glazes fit Cone 10 clays with very low COEs. 

But this doesn't explain Cone 10 glazes without boron and seeming excessive COE mismatches working out well, which leaves me wondering when does a COE mismatch create problems?  It's seemingly more than just the numbers alone. Perhaps glaze strength is the missing factor.

One example is Pete Pinnell's Cone 10 Weathered Bronze Green with a COE of 8.7 which seems a bad match for a Cone 10 clay with a COE of 3.46 like Laguna 373 Dark Brown yet the glaze fits well.

One of Pete's students discovered this glaze is wide-firing enough to also work at Cone 6, though I find it's more reliable at Cone 6 after adding 20% Ferro Frit 3269 (shown below). So it's become a popular Cone 6 glaze as well.