Not to confuse things, but there is "etching" to create contrast in color, and (deep) "etching" to create contrast in the surface of the barrel that one can feel.
From
Principe de Fabrication d'un Canon de Damas and translated from the French so possibly not clearly communicated
"The polishing-out of impurities with
sulfuric acid allows one control the perfect machining of the barrel. The action of vitriol brings out the steel in black and the iron in white wonderful patterns that repeat at every turn."
From
The Shooters Guide; Or, Complete Sportsmans Companion, 1816, Thomas Burgeland Johnson
http://books.google.com/books?id=5DQCAAAAYAAJ "It is now the custom to brown barrels; which is done by rubbing the barrel over with
aqua-fortis, or spirit of salt, diluted with water, and laying it by until a complete coat of rust is formed upon it."
Spirits of Nitre = It is obtained by the distillation of alcohol with
nitric and sulphuric acids.
Aqua Fortis =
Nitric acid. Nitric acid is a strong oxidizing agent. It reacts with metals, oxides, and hydroxides, forming nitrate salts.
Aqua Regia = a mixture of
nitric and hydrochloric acids.
Blue Vitriol = Alum and copper sulfate.
Side by side 4 Iron "Turkish" segments from
Buchsenmacher u. Jagdmuseum of Ferlach, Austria 
Acid etched Damascus - Twist
Lightly acid etched crolle can be
very attractive
It's probably easiest to contact him by PM here