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## thermal expansion of red iron vs black iron

### Paul Borian on fri 29 jan 10

i had an iron red i use on Glazemaster and was printing batch sheets of
some variations, which including substituting black iron oxide for the red
iron oxide i normally use. I tried several different combinations and found
that red iron oxide increases the thermal expansion considerably but black
iron oxide reduces the expansion.
i would not be surprised if i was the only one who did not realize this
fact - and i was wondering if someone could explain the reason for this
and/or provide a link to a site that provides details?
thanks,
Paul

### John Hesselberth on sat 30 jan 10

Hi Paul,

I assume you were entering the iron oxide as an 'ingredient' instead of =3D
as an additive. GlazeMaster only calculates expansion for the base glaze =
=3D
and then only if it has data for the expansion coefficient of the oxide =3D
of the material. Red is a relatively high expansion material--you can =3D
get to the list of coefficients from the main menu. There is none for =3D
black in the database. I am not sure if I tried to compensate for that =3D
by converting it to red or not--it has been a few years. But if I did it =
=3D
would only stay the same if there were equal numbers of molecules of =3D
iron in the red/black substitutions you made. And then it may not have =3D
been a terribly accurate thing to do.

Overall there are lots of assumptions built into calculation of =3D
expansion in any of the glaze calc programs. We all use the 'rule of =3D
mixtures' for example. It is kind of like using the perfect gas law. It =3D
work with simple systems, but when things get complex it breaks down. So =
=3D
this is probably a case where the expansion numbers are not necessarily =3D
very useful

Regards,

John

On Jan 29, 2010, at 3:46 PM, Paul Borian wrote:

> i had an iron red i use on Glazemaster and was printing batch sheets =3D
of
> some variations, which including substituting black iron oxide for the =
=3D
red
> iron oxide i normally use. I tried several different combinations and =3D
found
> that red iron oxide increases the thermal expansion considerably but =3D
black
> iron oxide reduces the expansion.
> i would not be surprised if i was the only one who did not realize =3D
this
> fact - and i was wondering if someone could explain the reason for =3D
this
> and/or provide a link to a site that provides details?
> thanks,
> Paul