relationship between speckled iron oxide and the last firing rate

[Didiho]

Dear all,

Having the same glaze putting into two different kilns - one is km 714 by skutt, and another is Falcon FL3.120, the ones put into  Falcon FL3.120 turns out with speckle. I wonder if it matters with the last firing rate above 1000c till my end temp (1222c).

When using km714, I firing with cone 6 medium firing. When using Falcon FL3.120, I'm firing with the below schedule:

 

120c/hr - 1063c

70c/hr - 1220c

 

I tuned the ending temp to 1220 as it seems due to the much larger chamber of FL3.120 than km714, the pyrometric cone shows it is a little bit over-fired.

The recipe mainly having a speckled issue is as below:

 

NS 20.37

silica 10.74

3124 22.56

Kaolin 29.23

Whiting 17.1

iron oxide 3

manganese dioxide 1.5

 

Thanks all.

[Callie Beller Diesel]

Is the clay body you’re using a porcelain or a stoneware? And is the speckling happening with any other glazes you’re using? Is it a really pronounced speckle, or is it really fine?

If you’re using a stoneware, it could be that impurities from the clay are bleeding through with the extra heat. This would be confirmed if the problem is appearing with other glazes.

If it’s only happening with this one glaze, and especially if the speckles are really tiny, the culprit might be the mesh size of either the manganese or the iron. If you can’t get a finer mesh size from your supplier, you could try ball milling the glaze.