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Wheel Head Leaving Dark Tarnish On Clay

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#1 Stone Spiral

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Posted 18 May 2017 - 03:27 PM

My wheel appears clean and silver, with no visible tarnish. However, when I pull my clay off the wheel, there is black/dark looking clay or slip where it meets the wheel. It's as though the wheel head is giving off a tarnish or the clay is reacting to the metal.


Will this cause problems when firing?

Is there anything I can do to prevent this?


I know I could throw with bats but I don't currently have any. I will go that route if necessary, but in the meantime, just looking to figure out what this tarnish is and if it's going to cause problems.


~* Roxy *~

#2 oldlady


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Posted 18 May 2017 - 09:33 PM

do not worry about it.  it is likely that glazenerd has a scientific description, i only have a working history with this.  it is simply a reaction between the clay and the wheelhead metal. 


you can see something like it when you look at a matte glaze on a plate and see the cutlery marks.

"putting you down does not raise me up."

#3 Dick White

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Posted 18 May 2017 - 10:00 PM

Yes, Glazenerd will be along soon and give us a better scientific answer. For the layman's answer, it is aluminum oxide. The wheelhead is cast and machined aluminum, and the clay (whether through chemical reactions or physical abrasion) will cause a slight amount of the aluminum to rub off onto the bottom of the clay or your throwing sponge as the wheel turns. It is harmless, will not affect anything in the firing despite its dark color now.

#4 glazenerd


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Posted Yesterday, 09:24 AM

Read this awhile back, and forgot to respond.

Aluminum "tarnishing" is actually an oxide layer that builds up over time; as it is exposed to other elements. ( sodium and potassium) primarily. Everyone has seen there new shiny wheel head tarnish in a short period. The oxide layer is primarily caused by the aluminum reacting to higher ph levels caused by the fluxes in the clay: actually it is a desired trait.
However, when you use a clay body with sodium as the primary flux ( mostly porcelain), it will break down the oxide layer, and bits of that layer will come off. Same thing happens in an aluminum barreled pugger. It should not present any problems because it is just a chunk of alumina oxide.
If you use a sodium based clay body regularly, then wipe the wheel head off with vinegar on occasion to stop the build up of the oxide layer.

*** so many reasons they should not use sodium in clay: this is just one more among many. But when sodium is 1/3 the cost of potassium, I do not expect changes.


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