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Thompson Finishes


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#1 rotorwing70

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Posted 14 September 2003 - 06:47 AM

I was reading the thread on WWII finishes and wanted to interject a finding of mine. I have a late 1928A1, which has all the late features. It is an as manufactured gun with matching serial number receiver and lower. As a little background I am a Chemical/Metallurgical Engineer and have many years experience in the metals casting and finishing area. I wanted to begin a restoration on my 28A1 but first wanted to confirm the factory finish coatings. At first glance I assumed the upper and lower receivers were phosphate coated "Parkerized". However, after microscopic examination of the surfaces there was no evidence of the characteristic zinc or manganese/iron phosphate grain structure always found with zinc or manganese phosphate coatings. The "Parkerized" gray appearance is the result of the parts being sand blasted prior to "caustic bluing" or "Duliteing" or "black oxide". This process produces a matte gray finish, which has the outward appearance of Parkerizing but lacks the amorphous crystalline structure of zinc or manganese phosphate coatings. Many manufactures have used a brushed or blasted metal surface to produce a non or low reflective surface finish. Most military S&W pistols have one of these two finishes. If any of you have an early original finish S&W Victory 38 it will have a like finish. After examining all the various parts on my 28A1 I am 99.9999% sure that there are no Parkerized parts. They are all blued with a caustic bluing process sans any flame bluing that may have been done on small parts like extractors or springs. The various shades of blue, gray and blue/black are related to surface preparation and metallurgy.
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#2 rotorwing70

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Posted 14 September 2003 - 01:22 PM

Thanks for your comments PhilOhio. Vietnam era Air Force PJ that got tired of being the first one out of the door and the last one back in. Only enlisted ranks in the PJís so I took a Warrant and learned to fly backwards later on. As for the 28A1 it does indeed have all the military acceptance marks and they are white metal where the stamps displaced the original finish. They have never been refinished over. The barrel is smooth and dark brushed blue/black while the Cutts is bright polished blue. The rivets on the brushed blue Lyman fixed L sight are in the white, not blued, indicating the sight was not attached to the receiver before finishing. The receiver is also finished under the sight. As for the gray color, Dulite is a caustic black oxide (bluing) process which produces a near black finish on low alloy steel like that used for most firearms. Winchester, S&W, AO, Marlin and others used it on their commercial gun and continued, to varying degrees, to use the process as war time production began. Winchesterís very early WWII Garand production, maybe as few as 200, was Dulite finished and are distinctive in that the remainder of the production run was Parkerized in varying shades of gray. When the sand blasted metal was finished with the Dulite process the normal black finished has a medium to dark gray color. Another common WWII weapon that bares this type finish is the Remington M11 shot guns. There were many commercial M11 pressed into service and had the usual bright polished blue/black finish. However, later military contract guns were marked "Military Finish" and had a blasted matte finish. On a microscopic level the difference between Parkerizing and caustic or flame bluing is quiet obvious. Phosphating (Parkerizing) results in the deposition of a crystalline substrate while bluing results from the chemically or flame induced "rusting" or oxidation of the ferrous metal. This oxidation is not really a coating as there is no deposition, only the oxidation of the ferrous metal on the surface. Parkerizing is known as a conversion coating process and does indeed deposit a coating on the metal in the form of amorphous metal/phosphate crystals.
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#3 rotorwing70

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Posted 14 September 2003 - 06:00 PM

PhilOhio,

You are right on with the auto body analogy. You are probably using something like "Ospho" which is a mixture of oxalic acid and phosphoric acid. The oxalic acid removes any remaining surface rust and the phosphoric acid reacts with the iron to form a iron phosphate complex which acts as a temporary rust inhibitor. In the case of Parkerizing, which by the way is a patented zinc phosphate process developed by Parker Chemical loosly based on an old British iron phosphate patent from the early part of the 1900's, the finish coating thickness can be regulated by time, temperature, accelerators and which companion metal is used with the phosphate. The most common phosphate conversion coating is zinc phosphate with manganese phosphate a close second. Zinc phosphate is commonly used by the automotive and appliance manufactures as a precoat for paint adhesion. Manganese phosphate is usually a thicker more dense coating and has recently found favor with the military for small arms and larger weapons and commercial users for such item as bolts and fasteners. Phosphate conversion coatings in themselves are poor corrosion inhibitors over extended time or exposure. They owe their corrosion inhibition quality to their ability to retain oil and sealers in the porous crystalline coating they form. Phosphated steel must be oil or wax coated immediately after removal from the vats to prevent flash rusting. The distinguishing features of the two phosphate processes are as follows; Zinc phosphate generally has a finer grain structure, is thinner, and is colored from light to dark gray. Manganese phosphate has a larger grain structure, is thicker, and is almost black in color. There are pretreatment processes to give the zinc phosphate a darker color like manganese phosphate but they are not generally used as it adds an extra step to the process that does not improve the performance of the coating.

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