Copper spots after glaze firing

[RWC]

Hello there

I have encountered a problem with a new batch of work. I'm using buff Oxidising St Thomas clay bisqued to 1000% then glazed with a white tin glaze and refired to 1240 with 40min soak. Electric kiln less than 12m old.

I have not changed anything in my production process and after consistent results for over a year the last batch of work has green spots in the glaze, maybe one or two spots on each piece of tableware (2-3mm diameter), whilst other pieces are fine.  It's definitely the clay not the glaze as I have drilled into the spots and you can see a spec of something under each one. My supplier thinks it's copper and says I must have introduced it to the clay somehow in the studio. The clay wasn't recycled and was used straight from the bag.

They have suggested there may be copper in my water supply from old pipework. We haven't had any plumbing done in the last few years.

Does anyone have any similar experience or thoughts to how likely this is, or any other suggestions!  There is nothing apparent in the bisqueware, it has only appeared after the glaze fire.

[neilestrick]

Can you post pictures?

Was all of the clay from the same production batch? Check the boxes for dates and production numbers.

I just don't see how chunks of copper could get into your clay body in your studio. Clay comes out of the ground, so it's very possible that the clay body manufacturer got some raw material that has some contamination of some sort. It happens, unfortunately.

[glazenerd]

From a quick search: St. Thomas is sold in the UK? If so, it appears to be a stoneware body according to Potclays?  Finding the contamination source could take time. In the USA; dark spots from iron, manganese,  and lignite coal particles is not uncommon. In the UK: not impossible to have natural copper sources. Not impossible that cross contamination occurred at the factory. Although copper would not be present in a clay factory unless they were specifically blending for a custom order.  Are there possible contamination sources in your studio space? 

It takes 2-3 times the copper in clay as it does in glaze to produce color: so copper in the water is most unlikely. For a year no problem, now a problem- so contamination occurred somewhere in the chain. You need to make a test tile from clay fresh out of the bag: unglazed. Clean hands, clean tools, clean surface when you make it. Fire it to peak 1240C (unglazed). If it has spots, then contamination occurred at the mill; if no spots- contamination occurred in your studio.

tom