Metallic Weirdness On Pots

[timbo_heff]

 Here's a strange result:

Used some old pieces of angle iron as grate bars in wood fire (cone 11+)

When opening the kiln there was some strange blue green stuff in the ashes left behind;

Unfortunately the stuff got on pots too:

It seemed to react especially terribly with an iron celadon (Mark's tenmoku with 2% iron)

but also made ugly stuff on shino and even unglazed wares.

 

Wondering if any one has seen problems from rusty metal in a kiln before:

The iron came from an old railroad bridge (1940's maybe)

The bars were rusty on the surface but still dense and heavy.

After the firing it was flaking in big chunks.

 

Could the problem be from just the iron?

I am thinking that some other impurity in the metal vaporized.

Perhaps the iron was poorly smelted in the first place and was giving off something like whatever is in the blue slag you find around old iron processing furnaces ?

 

 

 

 

[neilestrick]

We always used iron of some sort as grate material in every wood kiln I've used, and it always flaked during and after the firing. They only hold up so well under that kind of heat. But it never caused anything like that. We used angle iron, drive shafts, axles, whatever we could get cheap or free, but it never affected the pots. Even if you somehow got a chunk on a pot, it would look like a chunk of iron on it, dark and sometimes melting through the pot if it was a big enough piece. I'm thinking it's probably more related to the glaze and ash relationship than anything to do with the grate. Did you use the same wood source as always?

[timbo_heff]

Thanks Neil;

I would suspect a glaze prob but it was even on unglazed stuff and made bad on shino, tenmoku, clear, and celadon

 

I expect some gray and ugly in the front of the kiln, but the gray went deep into the kiln : way past the guard pots.

Insides of stuff look good and totally guarded things were ok.

 

"It"  seemed to get under some glazes :  tried and true shino / clay combos have unstable glaze that chips and flakes.

Other pots got some stuff like elephant skin : but firing was good : shouldn't be that flaw : it almost is like it got under the glaze and wrecked it from within.

It also seems to have gotten under the ash glaze itself so there is flaking /like shivering of ash glaze!

So perhaps it started vaporizing early in the firing and got onto the pots before the ash was deposited.

 

The wood was from a new guy but it was all nice standing dead mixed hardwoods and some ash. all clean and dry

The thing that makes me pretty sure it is from the metal is that in the ash pile itself there was the blue crusty stuff... I don't think that a glaze would vaporize enough to blow back into the firebox area and tint the ash pile (anagama style so the firebox is not separated from the wares but still I can't imagine it blowing back).

Mysterious and terrible. Definitely will not be using those again !

[Tyler Miller]

It's highly likely that Neil is right and it's something to do with the wood or ash, but it could be the metal. Angle iron is usually made of a36 or something similar. Structural steel is a compositional mystery as it is graded by properties not content. It could contain a little copper. Unlikely, but possible that it is the culprit. Rule everything else out first, as I can't imagine it would account for more than 1.5% of that angle iron, not enough to make a difference in my mind.

 

I always thought the blue slag was due to the iron in the fluxing agents/smelting stack walls, like a blue celadon. I get green slag the colour of a green celadon around the top of my heart remelter.

[neilestrick]

Was the blue crud on the iron grate? Or did it just look cooked and flaky? It seems like if there was something vaporizing out of the iron then it would have also had an effect on the iron itself. It also seems unlikely to me that there could be enough of anything in the angle iron grate to cause issues throughout the kiln.

 

Different woods can have a huge effect on the firing. I grad school a friend of mine did his thesis on firing with cottonwood. It was a bear to split, didn't give off much heat, created a ton of ash that clogged up the kiln, and made everything look like varnished concrete- gray and glossy. He sent some ash off to the lab and found that the cottonwood in our area was really high in potassium, hence the gray gloss. After some experimentation he figured out that with a high iron body (4.5%), and adjusting the firing to account for the heavy ash, the results were fantastic, especially when reduction cooled. It did some awful things with some glazes, though, that firing with pine didn't do.

 

Try a different grate in the next firing, with the same wood source and see what happens. If it is the metal causing the issue, it will be interesting to figure out what exactly is going on.