Collectors of antique edged weapons, especially those made in the Middle East, are well aware of the distinctive surface patterns of traditional Damascus steel blades. These patterns, which are the hallmark of such blades, are made visible through the use of a chemical treatment which reacts with the constituents (and their microstructure) on the surface of the steel. These patterns delineate the areas of differing crystal structure inherent in the blade during the initial making of the steel (called "Wootz") and its subsequent forging method. Such blades are highly prized. They should never be confused with forge-welded and "pattern-welded" steel blades, which are of an entirely different ancestry, and which have their own, distinct, merits.
Unfortunately, over the passage of time, many old Damascus blades have been subjected to abusive handling and harmful environments which have partially, or totally, obliterated the Damascus pattern. All is not lost, however, because the pattern can be rather easily restored.
Various writings on how the ancient bladesmiths achieved this pattern, describe highly different methods. One method described, which I would NOT recommend, is to suspend the blade for several days or weeks in a covered earthen pit at the bottom of which rests a resplendent deposit of cow dung. Many writings describe the use of "Zag", which is the Persian term for Ferric Sulfate. One reference, as I recall, stated that some Caucasian smiths suspended blades in a boiling solution of Zag.
The method that I have used with acceptable results to restore the Damascus pattern is as follows.
1. The blade surface must be smooth, clean steel with no rust spots. It must be made free of oily substances by heating and wetting in hot water and then scrubbing the hot, wet blade with a brush laden with a household powdered cleanser and then rinsed in hot water. When the water forms an unbroken film, the blade is considered to be sufficiently degreased.
2. Prepare a solution of Ferric Sulfate in water (say 2 tablespoons of the chemical to 8 oz. water.)
3. Place a plastic bowl in the kitchen sink to catch the solution.
4. The blade is heated in a hot water stream from the faucet. It is then held, point down, over the plastic bowl.
5. A squeeze bulb syringe is filled with the Zag solution and slowly applied to the blade, letting the stream run down the entire length of the blade and into the bowl. The solution caught in the bowl is not discarded, but is re-used as necessary. This process of heating the blade and then applying the Zag solution is continued, with repeated heatings, until a suitable coloration appears.
6. To inspect the effect. the blade may be wiped with a wet cloth or sponge. Step No.5 is repeated as necessary.
7. The still hot blade is rinsed well in hot water and dried with a paper towel. Any residual moisture is allowed to evaporate as the blade cools.
8. When completely dry, household oil (such as 3-In-One) is spread over the blade with any excess removed with a facial tissue. This emphasizes the pattern and protects the blade from rusting.
Depending on the characteristic of the steel, the color produced varies between greyish and brownish. Ferric Sulfate is available at any reputable chemical supply house.
A word of caution:Ferric Sulfate solution will also discolor any metal parts of the sink, which are not made of stainless material.
Under NO CIRCUMSTANCES use Ferric Chloride solution! It is highly corrosive and is used to etch the copper patterns on electronic printed circuit boards. I have seen a superb, rare shape, Turkish Kylij blade utterly destroyed with the overzealous application of this material.