Firing Range

[Summerss]

Hello, I have a new clay I’m trying and the firing range is cone 04 - 6. I’ve never used a clay that states a firing range” before. I’m wondering if this means I can firing the clay anywhere from cone 04-6 or if this means the bisque is 04 and glaze can be 6? 
 

thanks in advance!

[Hulk]

Hi Summerss,

Welcome to the Forum!

Could be the recommended bisque fire is 04 and the clay is fully mature at cone 6.
Can you provide identification/source of this clay?

Expect clay to become fully "closed up" - about as impervious to water as it will get, without starting to slump, melt, bloat, fizz or present any other typical overfired characteristic - at a particular cone. When fired much under the ideal cone, the clay will likely take more water, be less strong...
Some clays are more flexible that others in terms maturity range.
Some mid-range clays I've tried misbehave when fired much over cone 5; others tolerate a bit over cone 6 without issue.

That said, might be best to verify by test.
Fire the clay to 04. Is it fairly strong, does it "ring?" Does it take the glaze well - absorbs some water, but not too much?
From there, fire to target - try cone 6. Do the glazes come out smooth and shiny, no fizzy bubbles, no pinholes or other defects?
Fire some bare bars to test absorption with; weigh the fired bars, soak in water, pat dry, reweigh.

Put cones on each level. If part of your kiln fires cool, perhaps cone 5, good place to put more tests.
If cone 6 is too much, cone 5 might do.
Test!

The vendor may be able to provide some guidance.

[Summerss]

Great! Thanks for the recommendations!

[Roberta12]

10 hours ago, Summerss said:

Great! Thanks for the recommendations!

@Summerss I am curious as to which clay you are using?

[Summerss]

The link to  the clay I’m referring to is below. It’s from a local vendor. https://rovinceramics.com/products/ro-82m-terracotta-with-fine-grog?_pos=1&_sid=94c8d131d&_ss=r

[Hulk]

Nice color!

From the linked page: "Absorption at cone 06 is 10.8% and at cone 6 it is ≤0.1%"

   ^ I'm interpreting that as
                    a) cone 06 is the lower end of the bisque range and
                    b) cone 6 is near the upper end of the glaze fire range

For functional work, low absorption is better.
One percent or less is good, very good.

[Summerss]

This is very helpful! Thank you! 

[neilestrick]

Terra cotta has a fairly wide firing range. The addition of grog extends that by a couple of cones. It tends to get more brittle as you fire it hotter, and based on the absorption numbers they show, I would say the top temp that is truly functional would be more like cone 3 or 4, not 6. Typically terra cotta is fired to low fire temps, like cone 04-ish, but I do know people who work with it at cone 3. It gets darker as you fire hotter, and you won't find commercial glazes for cone 3. You'll have to decide what's going to work best for you among all those factors.