metal color change

I need some advice.
I am taking stamped metal parts (nickel silver) and heating them up to about 450 - 500 degrees. Many of them change color from a shiney
silver to a bluish color. This is the effect I want.
Lately, many of the parts are not changing color. When stamped, we use a light coating of kerosene for die lube. Sometimes we run fairly dry.
IS the color change a result of the light kerosene? Is it a function of temperature? What can I do to ensure that all the parts turn bluish?
Thank You,
Wayne
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Certain that the Nickel alloy is the same ? A tiny gold delivers deep purple.
I'm not a metal expert, however I suspect the color is from the copper.
Are you cleaning before on all of them before heat ?
The color is from oxidation of the copper I rather expect.
You might not have an oxidation heat but a reduction heat if done by flame.
Those are my thoughts.
Martin
Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net TSRA, Life; NRA LOH & Endowment Member, Golden Eagle, Patriot's Medal. NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member. http://lufkinced.com /
snipped-for-privacy@yahoo.com wrote:

-
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Martin,
We are using a small oven. Heats to approx 400-450 maximum. Cleaning parts with kerosene before heating. When the oven was new, it was getting up to 500 dgrees and it seemes the color change was more consistent. Can the issues be the reduction in heat capacity? At what temp does copper start to turn blue?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
The copper and if it has gold - it has to be in an oxidation state. The blue is copper oxide.
Sounds like the bad dude is the Kerosene and the oven not at 500.
The Kerosene is an oil base and that coats the metal preventing oxide. Likely the high temp burnt it off.
Try washing with something else. And then try it.
As to the temp - maybe you can blanket (high temp) the oven - or box it and increase the temp by lower loss.
Try with only a different wash.
I can't find an oxidation temp for metals. I'll continue to look.
Martin Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net TSRA, Life; NRA LOH & Endowment Member, Golden Eagle, Patriot's Medal. NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member. http://lufkinced.com /
snipped-for-privacy@yahoo.com wrote:

-
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.