Hey everyone, I have been working real hard researching any and all mention of Colt finishes. I found these pics posted elsewhere. One with the oven photo of the worker close up is in the book "A Century of Achievement" by Colt Firearms. I have a copy of the book on the way I bought so I will confirm. Anyway these are the Colt, and Smith and Wesson American Gas Furnace rotating ovens.
The top photo obviously has the large factory leather belt on the sides of each for power; the Photo 47. was noted as Smith and Wesson operation and you can see the side mounted electric motors and switches. The three vertical coupling on top appears the rear one has a gauge I believe to be pressure gauge because the pipe connection appears to be sealed.
As posted by: jplower http://www.coltforum.com/forums/single-action-army/100875-colt-bluing.html
NOTE: oakum or tow is a tar coated rope probably similar to burlap sack. The whiting is calcium carbonate or chalk which to my understanding rids the surface of organics material which may be in the charcoal. Bone ash char also can have a percentage of calcium carbonate and may activate the carbonization of the hot parts surface as well. Today bones ash is sold by Ebonex Corpation http://www.ebonex.com/history.html
In the rotating ovens the charcoal and bone ash mixture is reversed tumbled across the parts. The reel or rack of parts being put in the oven rotates one direction and an outer chamber drum that tumbles the charcoal and bone ash goes in the opposite direction of the reel / rack of parts.
Also, all sources indicate the parts whether in the trough or ovens is evacuated of oxygen around the parts and exposed to carbon. By burying the parts in the charcoal trough oxygen is reduced as the thermal heat rises near the parts placed at the top. I imagine as you de rack the parts to wipe with the oakum and whiting, would take some time.
I only have examples in the 149,000 serial numbers so please post if you have later examples comparing the later patina differences you will see in the photos below. The time period is 1916 to 1917 and it is easy to confirm the different finishes. I have confirmed form multiple metal finishing sources that the "black" finish is direct result of the charcoal carbon oil burning smoke during the heating process. The later oven models will reflect black (smoke layered carbon) at one angle in the light; then the silver blue (tempered layer from temperature range). I tempered some rolled steel samples in my kitchen oven easily the silver blue tempering layer, but only got the black addition when rubbing the hot sample with motor oil that burns off. There is no doubt in my mind that the carbon exposures blackens the surface in the tempering process of the Colts.
Below are photo examples of the trough ovens, verse rotating ovens finishes of Colt 1903s. The trough models are very black, the oven models very black / silver blue variation as pistol rotated in the lighting. High polish models have no visible sanding polishing grain lines as shown below.
The following pics are Colt 1903 Serial # 14491 trough finished with / without overhead lighting: NOTE the perfect reflection of the light lens diffuser pattern.
The following pics are Colt 1903 Serial # 146419 having a slide and frame patina variation. Trough finished with very meticulously custom high polish with / without overhead lighting. Interestingly, with overhead lighting on, the finish looks identical to a nickel finish except nickel finishes do not turn black when you turn off the overhead light.
The following Colt 1903 Serial # 472958 is American Furnace rotating oven finish with / without overhead lighting. NOTE: The sanding polishing grain lines are clearly visible around the serial number which is a regular polish finish and not a high polish mirror finish.
Also this one is for sale on Gunbroker if any interested ending 1/14/2018 9:45 PM, I am EXRINGS1 seller.
The following engraved Colt 1903 Serial number 148705 is in my opinion 100% tempered blued very likely without any carbon charcoal smoke. This finish can be done in electric kilns. It is possible there was a hot box of charcoal which oil could have been injected to smoke fume. Basically your kiln chamber is sealed which you have a small metal box with charcoal inside the oven base then once you reach a certain temperature or time frame a tube allows oil to be squirt into the activated charcoal and smoke fume the chamber and pistol laying on the carbon layer.
The Colt's with the regular polish to high polish finish are very difficult to photograph. The above pistols were all photographed with 3- 100 watt LED Daylight bulbs in clamp lights with diffuser material ( white T-shirt / and lexan light lens used in fluorescent light boxes ). Due to reflection of light, and the actual absorption of light due to the carbon layer the auto features must be forced by shutting off ISO sensitivity on the Nikon DX 3000 shooting on aperture priority. I shoot on aperture to get the best focus on the edges of the center light focus metering and go highest aperture as possible . Many shots drop 1/60th speed even though there is plenty of light with 3 lights. Often the 1/60th speed is too slow to prevent camera shake and some blurring.
I hope everyone enjoys seeing a photo comparison with the same lighting and photographing set up of the Colt 1903s tempered carbon bluing. 8' clamp lights work well and save space. Be sure to have all light bulbs in the room and the clamp light bulbs all matched with Daylight bulbs. A different light source type light like incandescent will make a brown light reflection. I shoot in my garage and I changed out every single light bulb including the overhead door operator to all the same Daylight LED. This light source works great especially with cell phone digital cameras. Also spend the extra money on 75 to 100 watt verse 60 watt for greater lighting.
I hope this information demonstrates that there were some rarer examples of the Colt 903 that are known as high polish mirror blue. I am still searching for high polish mirror blue examples of the Colt 1908 but have not found any to date. A high polish mirror finish must reflect a full mirror type reflection under lighting to be a high polish mirror Colt. The overhead lighting clearly demonstrate the reflection created by high polish mirrored finishes. Plus the reflection off the surface of the surrounding objects, clamp light diffuser, my skin, etc.