description of the term straw

[Ragnar]

As a new owner of a Luger and member of this community may I ask what is meant by the term straw?

[mrerick]

It's a type of Nitre / Fire bluing oxidized metal finish that you find on DWM, Erfurt and early Mauser / Kreighoff Luger small parts.

Do checkout our FAQ document.

[DavidJayUden]

I like to describe it as the color of metal that is heated to just a certain temperature. Sort of a golden/yellow.

[Eugen]

Ragnar, welcome to the forum. I think you got two great responses to your question above.

If you haven't already done so, check out the FAQ section. It is more of a primer and contain so much info that may interest you. I find it immensely helpful.

[DonVoigt]

Straw has nothing to do with nitre bluing.
It is the color reached when steel is heated to around 430 degrees F.

[cirelaw]

Besides being pretty, what other purpose was it?

[DavidJayUden]

Good question, Eric. As far as I know the metal is no more protected or tempered than non-straw colored parts.
But next to rust blued steel, it sure does look nice. I just picked up a 1908 Mannlicher-Schoenaur carbine and the metal is all nicely rust blued with just the right amount of strawed parts. Plus excellent wood-to-metal fit and screw slots so thin that no modern screwdriver will fit them.
Ah, the good old days...
dju

[mrerick]

See: https://en.wikipedia.org/wiki/Bluing...)#Niter_bluing

"the colours will range through straw, gold, brown, purple, blue, teal, then black."

Obviously, this bluing process was not used for the traditionally strawed parts of a Luger, but it was used on the grip screws.

[Ragnar]

thanks for the explanation gentlemen!

[DonVoigt]

Originally Posted by mrerick

See: https://en.wikipedia.org/wiki/Bluing...)#Niter_bluing

"the colours will range through straw, gold, brown, purple, blue, teal, then black."

Obviously, this bluing process was not used for the traditionally strawed parts of a Luger, but it was used on the grip screws.

I'll keep arguing that "nitre" is not a bluing process; the "nitre" salts are only used as a not so convenient way to heat the "white" metal.

The color change arises from the heat- not the salt; the same result can be obtained by heating in air, or in any other medium- sometimes even molten metal has been used.

As the Wiki said, the object must be watched carefully as it is heated in the very hot salt bath, as the color will change from "white" to "black"(actually more grey)- if left in too long- passing through the various colors on the way.

This chart has been posted many times:

[Kyrie]

Originally Posted by DonVoigt

Straw has nothing to do with nitre bluing.
It is the color reached when steel is heated to around 430 degrees F.

Done is entirely correct.

Straw (and fire blue) is not a finish; it's a byproduct of the flame hardening of low carbon steel to change the crystal structure of that low carbon steel to improve its resistance to abrasive wear. Flame hardening, regardless of whether that hardening is actually done by open flame, closed oven, immersion in molten metal, or some similar technique, produces a part that is hardened through and through. The surface color is determined by the heat to which the part is raised. That color will quickly oxidize to a muddy brown unless intentionally preserved.

Only small parts are flame hardened. On a Luger, these small parts include the safety, locking block, trigger, ejector (which usually has the color buffed off), and the hold open (deferentially hardened to fire blue). Flame hardened parts on a Mauser C96 included the trigger, safety, bolt stop, extractor, and rear sight lock bar.

The reason straw color disappeared from Lugers in the late 1930 is the same reason rust blue disappeared at the same point in time; at that time Mauser discovered a better way to heat treat small parts than flame hardening, and a better way to do surface carbonization than case hardening. For a detailed account of this new process and the ways in which it was a manufacturing improvement see Richard Law's "Backbone of the Wehrmacht (The German K98k Rifle 1934 – 1945)".

[DavidJayUden]

So, Don and Kyrie, is the metal part, once heated and straw colored, in any way preserved, hardened, strengthened, tempered or in any other way "improved" by the straw process? Or is it just colored?
dju

[DonVoigt]

Originally Posted by DavidJayUden

So, Don and Kyrie, is the metal part, once heated and straw colored, in any way preserved, hardened, strengthened, tempered or in any other way "improved" by the straw process? Or is it just colored?
dju

Yes, it can be in "improved" condition. The original color was a result of "drawing" a previously heated , quenched or air cooled part, which was hardened- to a lower temperature to relieve stress and impart some property to it.

The straw on small parts at a low 430 degrees, still leaves them hard- but stress relieved. The "blue" of the small springs, like the sear spring and the mag spring is there due to the part being heated to a higher temperature to "make" the spring.

If you look at a clean early hold open, you will see a spot of stress relief by heating which shows a rainbow of color in about 1/4"- from yellow to brown/red to bright blue- then on the other side of the blue, back through the brown/red to the yellow straw. This leaves the major part of the hold open hard but tempers the heated area to relieve stress and prevent cracking.

The color is/was originally a consequence of the need to draw and not for decoration.

[Kyrie]

Originally Posted by DavidJayUden

So, Don and Kyrie, is the metal part, once heated and straw colored, in any way preserved, hardened, strengthened, tempered or in any other way "improved" by the straw process? Or is it just colored?
dju

Yes sir, and this takes us back to the nature of the material used to make firearms, especially in the late 1800s to the mid to late 1900s; low carbon steel.

Low carbon steel was the wonder material of the late 1800's; cheap and easy to make, soft enough that it was easily workable without wearing out cutting/abrading tooling, and capable of being surface hardened by surface carbonization (usually case hardening), or by changing the crystal structure of the steel harden it.

Low carbon steel is soft enough to be easily shaped, but left soft has poor resistance to abrasion. Relatively large gun parts that would be subjected to abrasive contact with other parts were case hardened to have a thin "skin" of high carbon steel (which is what the surface carbonization process does). This surface hardness was generally "file hard" meaning a file would not bite, but would skip across the hardened surface. Taking a file to a part that has been surface hardened feels like trying to file a piece of oiled glass.

The process that produces straw and fire blue colors is different from surface carbonization, but like the case colors produced by surface carbonization, the straw and blue colors are a by-product of the hardening process and not a goal in and of them selves. More on this later, when I get some time....

Hardening only the surface, and leaving the core material soft, leaves the part capable of take shock without cracking or shattering (preserves the immense ultimate strength of the core). The surface hardness allowing the bearing parts to perform many cycles with little to no appreciable wear. Surface hardened parts of a Luger include the frame, barrel extension, and the toggle links. In a model 98 Mauser rifle, the major parts subjected to surface carbonization include the receiver and bolt body.

Case hardening produces, at the quench, surface colors. These colors are usually just muddy brown or grey, but it was discovered the additition of different elements to the material used in the case during surface carbonization would produce different colors - some of those colors were just stunning.

These colors produced at the quench would quicky just fade to muddy brown/grey, and people worked to find a way to fix the colors so they did not change. That effort was successful, and "case colors" were born.

The production of different colors at the quench became (and still is) something of an art form, with the specific materials added to the pace being a closely held secret. Companies that provided case hardening services were commonly located close to slaughter houses, as animal bone was one of the major components of the pack. Things added to the pack to produce colors like green, yellow, and royal blues included some passing strange things, including salts produced from urine.

The lock plates of Spanish side lock shotguns are classical examples of some of the colors possible as a side effect of case hardening and are true works of art. And, in this world of contradictions, it's not unusual to find the inside surface of such lock plated have had the case colors buffed off. Go figure.

In any event, the colors produced by surface carbonization are a by-product of the conversion of the surface of the part to high carbon steel. Discovering how to make beautiful, and fix, these colors doesen't make the part much more corrosion resistant, but does increase the price point at which the firearm can be sold.

[Kyrie]

Quick recap:

The above description of surface carbonization pertains to the heat treatment of low carbon steel parts that require a very hard surface to resist abrasion, but also require a fairly soft core that allows the part to take repeated shocks without failing by cracking, splitting, shattering, etc. Examples of such parts in Lugers include the frame, barrel extension, and the toggle train parts. In Mauser C96s, example parts are the frame, barrel/barrel extension, bolt, and hammer. In Mauser Model 98 rifles, example parts include the receiver and bolt.

In terms of those parts that show straw or fire blue color, these are (relatively) small parts that also require a hardened surface to resist abrasion. But these parts have characteristics that make surface carbonization a poor choice to attain that hard surface. Examples of such parts include safety levers, locking blocks (in Lugers), bolt stops (in C96s), tangent rear sight adjustment bars, triggers, extractors, and so on.

The major material characteristic that make surface carbonization a poor hardening choice is the relatively small size of the parts, which means the part isn’t big enough to retain a soft core after surface carbonization (meaning the entire part become high-carbon steel). Additionally, these parts don’t usually experience the repeated mechanical shocks that make a soft core desirable.

The net result of all this is these parts are not made hard by making them, in part or whole, high carbon steel. Rather hardness is imparted to them by changing the internal crystalline structure of the steel of which they are composed.

Caveat: this is a subject that can get fairly complex fairly quickly, so I’m going to try to keep it simple at the expense of detail.

All steel, including low carbon steel, takes the form of steel crystals called "grains" or crystallites. Generally speaking, the smaller these crystals are the shorter the boundaries between crystals, and the harder (and more brittle) the steel will be. Conversely, the larger these crystals are the longer the boundaries between crystals, and the softer (and more ductal) the steel will be. The size of these grains in a piece of steel can be changed by heating the steel; the hotter the steel (top end around 1500F – 1600F) the smaller the grains.

If a piece of steel is heated sufficiently to change the size of the grains from large to small and then allowed to cool slowly the grains will revert to their original large size as the object cools. If the steel object is cooled suddenly (“quenched”) by dipping it in water (or other heat sink material) the small size of the grains is generally retained.

Our small gun parts composed of low carbon steel are heated to the 1500F – 1600F temp and then quenched. This leaves them extremely hard, but also too brittle to be useful. We want to moderate (i.e. “temper’) the brittleness while retaining most of the hardness, and this is done by re-heating the part to some specific temperature range. Our target temperature re-heat range is determined by how much hardness we want to trade off for how much reduction in brittleness.

The surface of piece of hardened steel will, when being tempered in an environment where oxygen is present, oxidize (essentially, rust). The color of this surface oxidation is largely determined by the temperature to which the piece of steel has been heated.

Light straw color appears at about 350F, moderate straw color at about 400F, and dark straw at about 440F. These are the tempering temperatures commonly used on pre-1938 Luger triggers, safeties, locking blocks, etc.

Dark blue color appears at about 500F. This is the tempering temperature for the leaf spring used to provide pressure on the Luger’s magazine release button, and on the extractor and safety of a Mauser C96.

These are the heat treatment colors (really, the heat treatment temper colors) found on Luger small parts, and the origin of those colors.

A final note, concerning the corrosion resistance properties of heat treatment colors. Heat colors do provide some small, and unintentional, protection from additional surface rust. This protection occurs because the colors themselves are a form of rust, and that rust locks up the surface iron atoms in a form of iron oxide, making those atoms unavailable for further oxidation. But, again, this is an incidental effect and not an intentional attempt to produce rust resistance. Ultimately, there are other, more effective and less labor intensive ways than heating and tempering to prevent surface rust (among those ways, and the most commonly used way, are the surface oxidation techniques we know as “bluing”).

[DonVoigt]

Two great posts Kyrie, lots of detail.

For those who want the short answer, see post #13.

[DavidJayUden]

Moderators, can we save this thread somewhere?
dju

[Kyrie]

Originally Posted by Kyrie

--- Snip ---
Case hardening produces, at the quench, surface colors. These colors are usually just muddy brown or grey, but it was discovered the addition of different elements to the material used in the case during surface carbonization would produce different colors - some of those colors were just stunning.

These colors produced at the quench would quickly just fade to muddy brown/grey, and people worked to find a way to fix the colors so they did not change. That effort was successful, and "case colors" were born.

The production of different colors at the quench became (and still is) something of an art form, with the specific materials added to the pace being a closely held secret. Companies that provided case hardening services were commonly located close to slaughter houses, as animal bone was one of the major components of the pack. Things added to the pack to produce colors like green, yellow, and royal blues included some passing strange things, including salts produced from urine.
--- Snip ---

Let’s do some pictures.

Taking advantage of the need to do surface hardening by case heat treatment to produce beautiful case colors is an art form. I suspect that art form reached its peak in the production of artisanal side lock shotguns.

What follows are some pictures of the colors that were produced by master Spanish shotgun makers, as the case hardened the lock plates of Spanish side lock shotguns.



Here is a close-up:



The uninitiated will sometimes mistake these case colors for a kind of bluing; they aren’t. These case colors were produced at the instant that the newly hardened lock plates were dumped from the heating case into water to quench the surface carbonized lock plate.

The temperatures at which surface carbonization is attained will frequently warp the lock plates, and the warped lock plates must be beaten back into proper shape. Remember the earlier discussion on leaving the interior steel soft so the part can endure repeated shocks without shattering? This reshaping of warped lock plates is an example of what that soft center makes possible. If the locks were hardened through and through, they would shatter rather than reform back into the proper shape.

Here are a couple more examples of case color:

From an Astra side lock 12 gauge:


From a Gregorio San Martin 20 gauge side lock:



Case colors, like the heat straw colors, wear fast and disappear with handling. Here is an example:



Many, many shotgun people don’t understand the nature and origin of case colors and suppose case colors to be a finish, like bluing. It’s not unusual for these folks to ask about the restoration or re-coloring of case colors. That’s made faking case colors into something of a village industry.

Indeed, the ability of gun makers to produce attractive case colors at the end of the case hardening process is a dying art, and lower priced shotguns frequently are produced with (relatively) cheaply done fake case colors that are produced chemically. Here is an example of a lock that had fake case colors applied: