How to tell metal temperatures?

Jewelry Discussion
#1

Hi All,

I want to know how you are all testing the temperatures of your
molten metals during the casting process; I haven’t had much luck
finding any resources on this topic.

Any advice would be greatly appreciated.

Regards,
John Watson

#2 
I want to know how you are all testing the temperatures of your
molten metals during the casting process; I haven't had much luck
finding any resources on this topic.

If you melt with a torch, you do it simply by eye and experience.
The metal, when ready to pour, is fully liquid and fluid. Flux
residues on the surface sometimes are highly mobile or spinning at
certain temps. Stuff like that.

If you melt with a melting furnace, the temperature of the metal is
whatever you’ve set the furnace to achieve, within the limits of
accuracy of the furnace, and assuming you let the metal fully melt
and reach equalibrium with the crucible, etc.

More sophisticated casting machines which use built in melting
setups can simply be programmed, same as the seperate melting
furnaces, for the desired casting temperature of the metal.

If you need more accuracy or the ability to actually measure the
temperature of the molten metal, either in a furnace crucible or a
torch melt crucible, the best way is an optical pyrometer. Point it
at the molten metal and it tells you what the temperature is.

Peter Rowe

#3

John-

I just look. If it’s molten and it splashes like water when I swish
it around in the crucible, it’s ready.

I have no idea at what temp. metals melt. It’s all feel for me.

Jo Haemer
www.timothywgreen.com

#4 
I want to know how you are all testing the temperatures of your
molten metals during the casting process;

Well, John, I personally lick my finger tip and hold it up in the
wind…;} Seriously, I don’t know what the temperatures are without
looking it up, and I really don’t care, either. That is, the
numerical figure in For C. I just say, “That’s it, right there” and
let it fly…

But we’re just a podunk, small time caster. When you go upwards in
equipment you get into induction casting, calibrated sensors,
integrated pyrometers and electronic melting and all that sort of
stuff - if you’re casting a hundred flasks each and every day then it
matters very much. If you’re curious or need it, you can also just
buy an IR pyrometer (Google pyrometer) that will tell you the
temperature just by pointing it at the metal.

They’re not cheap…

#5 
If you need more accuracy or the ability to actually measure the
temperature of the molten metal, either in a furnace crucible or a
torch melt crucible, the best way is an optical pyrometer. Point
it at the molten metal and it tells you what the temperature is.

You need a very sophisticated optical pyrometer to accurately measure
molten metal. There are a whole handful of issues that contribute to
errors when measuring molten metals with optical pyrometers but low
emissivity may be the worst. Optical pyrometers measure the infrared
light emitted from a surface. The more reflective a surface is the
harder it is to measure it as there is less light emitted from
reflective surfaces, molten metals are very reflective. To measure
these low emissivity surfaces accurately you need what is called a
two color pyrometer. A standard one color optical pyrometer can be
(and usually is) significantly off when measuring molten metal. Two
color pyrometers are very expensive (many thousands of dollars).

Jim

#6

Your eyes will tell you.

Since my first time that I saw somebody melting gold ( I was 10-11
years old) I got fascinated with that fantastic sun like color. And I
remember my dad telling me a story :

The ancient Damascus artisans when have to make a sword, they start
to work in a very specifying time, in order to be almost ready when
the sun was red, then they sacrifice an animal, and…that was the
secret, the sun, the blood and the metal should have the same color.

Probably I missed parts on that story, but the point is COLOR. Color
indicates temperature ( lake on Kelvin scale) and even if you have a
special thermometer( that should be very expensive) you will not have
enough hands to hold it + torch + other tools. You must recognize the
exact color.

Vlad - www.braziliangems.org
BRASIL

#7

Thanks for the replies,

I have found the dual color pyrometers on line and ya they are into
about $3000 to $3500 for one that can read up to 2500C, a little much
for me just now.

Although this would be ideal for gauging the specific temps when
casting.

Regards,
John Watson