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Discussion Starter · #1 ·
gentelmen;; does anyone have any data regarding the effects of ultra violet light on color case hardening and straw case hardening? i can find nothing in the forum "archives".. any data appreciated. john l.lockwood
 

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No, I know that a strong directed light, such as a flashlight with a tight beam will show the patina better than just light. Ultra-violet light is good for making newer items, such as new thread stand out (it appears to glow).

Ed
 

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Ed,

I think John is asking about the possible effect of UV (in light such as sunlight?) in causing color case hardening and straw to fade. It is a known phenomenon in color pigments such as paint and color photographs, which is why museums display paingings and photos under subdued lighting and don't allow flash photography.

Quite a long time ago Thor proposed that light was a cause of straw fading, but it was a supposition, not as specific as identifying a particular part of the bandwidth.

--Dwight
 

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Discussion Starter · #5 ·
hello all; thank you for your responses. i have been keeping my collection in the dark for fear of diminishing the color case hardning colors, especially on my collection of M-99 savages. perhaps there was nothing much to be concerned about all along.--john--
 

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John,

I was intending to indicate that 1. UV fades colors; and 2. light -may- cause straw to fade. I understand your concern about your collection, and do not suggest that you make any changes or reduce your concerns on the basis of my comment.

--Dwight
 

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Strawing fades quite fast in sunlight. Violet/blue takes a lot longer. I lost the colour on my newly strawed Luger trigger within one year but the extractor has never been re-blued and the colour is still strong.

Patrick
 

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Discussion Starter · #8 ·
i raised the question for two reasons;;;first was that i thought it was an old wives tale, but secondly ;;i have six nice C-96's and an astra 900 that all exhibit the same phenomenon-- the blue is nice, some going to a deep purple, but virtually all the straw has faded to almost white.
 

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Strawing is not all that difficult to renew and a lot of people do it at home in the oven. You can´t do much wrong, as if you leave the item in too long at too high heat and it turns too dark, the colour can be removed with metal polish. To begin with, the item must be clean and grease free. When the correct yellow colour has been achieved, remove the item and dip it in cooking oil. I hold the item over a gas ring until it´s ready.

Patrick
 

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Dear Patrick:

For us slow learners, could you please advise how long you leave parts in the oven and what type of cooking oil? When you say you hold part over gas ring I would say a gas burner?

Regards,

George
 

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The straw color is the result of surface oxidation. The colors come and go through a temperature range, light straw is one of the lower temperature ranges, followed by a bright blue, a purple a dark blue and a black. The colors are formed by various mixes of ferrous and ferric oxides that are fairly consistent with temperature and the oxygen availablity. A heavier coat can be produced by longer temperature exposure.

I do not have my books with me or I could give you the temperature ranges. Color case hardening takes place when the part is buried in a chemical powder containg carbon, like bone meal or powdered charcoal, and some additional agents to make the rate of oxidation different on diffent areas. Carbon is driven ointo the surfface making it harder and the uneven distribution of coloring agents, like ferric cyanide, give the mottled color.

The color shift with temperature indicates how hot a piece has been and was used in the tempering process to judge when enough hardness had been "drawn" back out of a hardened piece to achieve the desired hardness and flexibility. Blue or purple are the right color for spring steel, note the color of the flat springs in the Luger mechanism.

It is possible that the straw color in the various luger parts had to do with a low temperature tempering or it may have been only decorative. The blur/purple spring colors are part of the tempering process.

The oxidative states which contribute to the colors can be unstable, especially for those oxides formed at low temperatures. Energy from UV, or infrared or even high environmental temperature could cause the states to relax back to the light gray. The higher heat oxides are more stable.

To do the straw process, you can raise the part to the correct temperaature in an oven or in a gas burner flame. About 400 degrees is a good place to start in an oven. If using the oven a coat of motor oil or lampblack on the part will help limit oxygen access which generally makes the color more intense. Put in a few test pieces of scrap to learn the oven temperature and time need for the color you want. Lower temperature and longer time is best. Using a live flame like a gas burner creates and oxygen poor environment around the part so a coating is not needed. Without the coating you can watch the color form. Immeadiately on removing the part from heat you must quench it in oil to cool the part and stop the chemical process. Otherwise it will revert to grey or progress to a blue. Be very careful. You may seriously change the hardness of the part.

Blues tend to age blacker, straws tend to fade. My observations of color case hardening is it tends to fade.
 

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Discussion Starter · #13 ·
hello heinz and thank you for that very enlightening information. i and i'm shure others got a good piece of education from it. one observation-- if one has an 80 or 90 year old luger with say 80% straw remaining, it seems logical that the gun has spent many years in the dark. i plan on keeping light exposure on my collection to an absolute minimum. --john--
 
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