Heat Bluing

Heat bluing of steel was originally used in watchmaking as a form of corrosion resistance. Because of the time & skill required this then also came to be regarded as a mark of superior quality.

The process of heat bluing applies a thin oxide layer to the treated steel part, with the thickness of the oxide layer dependent on the temperature to which the part is raised. The oxide layer on the finished steel part is clear, with the blue colour then resulting from thin film interference.

A first portion of the originating light wave reflects off of the top surface of the oxide layer, with a second portion of the light wave refracting through the oxide. The second refracted light wave reflects off of the steel substrate & then recombines with the first reflected light back at the oxide surface. When the two light waves recombine they are out of phase. The thickness of the oxide layer determines how far the two light waves are out of phase, & hence the colour of the combined light.

Since the thickness of the oxide layer is a function of the temperature to which the part is heated, controlling the temperature then determines the colour of the finished part.

The steel is grey, the oxide layer is clear, but the light observed by you the viewer is blue. Science!

Achieving a uniform colour on the finished part will only happen if the oxide layer is of uniform thickness, so success in the finished work is dependent on the preparation of the steel surface. The steel part must have a high polish applied, & be very thoroughly cleaned of any residual polishing compound.

Going back to the 1700's, watchmakers would traditionally use a spirit (alcohol) lamp & a brass plate to transfer heat to the steel part being blued. I have done this in the past, although my preferred method now is to use a copper plate on the stove-top. Other watchmakers will have their own preferences, but I find that the gas heat on the stove allows for good temperature control.

At around 135C the steel part will take on a straw colour, this is the first indication that the oxide layer has started to form. Starting at 135C & up to 170C, the colour effect will range from straw to golden to copper to purple, & then to deep blue. For temperatures beyond this, the oxide layer becomes too thick to support the thin film interference effect & the resulting colour of the reflected light goes back to the clear grey of the steel.

One can always put more heat in, but you can't take excess heat back out once you have overdone it. Considering this, I will want to take my time with the first part being treated, ease my way up to the desired temperature for the copper plate, & then hold it there. This can take a few minutes for the first part, but then once the copper plate is at the appropriate temperature the desired oxide thickness will be achieved for just a couple of seconds contact with the the plate for the parts that then follow.

It's up to the individual watchmaker "how blue" to make the resulting part. Some prefer the darker purple in the 155C - 160C range, some will want indigo blue from the 165C - 170C range, while others will want to push it past this into the 175C range & get the lighter blue. I like the effect of the deep indigo blue, & this is probably most consistent with what the old masters of European watchmaking would have produced.

With correct surface prep & careful application of heat the eye perceives light reflecting off of the upper oxide surface, but with the colour originating from the lower steel surface beneath. This can have the appearance of looking down into the colour, in a way similar to the paint finish on some very high-end cars.