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I'd like to finish a lamp I made of brazed chain, and so far my reading has uncovered blackening with oil (linseed or motor), or with apple cider vinegar. What is the difference between these two? The vinegar seems to create a thin layer of Ferric Acetate on the steel piece, but how does the oil quench actually function as far as forming the protective layer?
For indoor use, are there any other finishes I can do at home worth looking into?
Treating with vinegar (or other acids) tends to form a dark grey patina, depending on how it is applied it can also also show variations in shade. More viscous things like mustard and tomato sauce can be dabbled or stippled on to create a distinctly mottled effect. Acids also work well to strip off or at least dull bright zing plating and galvanising.
This finish needs to be dried and sealed with wax, oil, lacquer etc fairly quickly (ie within minutes) or it will start to rust. Also because it is somewhat porous it can be a good base for tinted oils and waxes.
Oil blacking works by polymerising and partly burning the oil. Here the temperature is quite important as the metal needs to be hot enough to blacken the oil without burning it off completely. If you watch the oxide colours as it heats up the point when it goes from deep blue to grey is usually about right. This process is pretty similar to seasoning a cast iron pan. You can either dip the piece in a container of oil or paint it on with a rag (don't use a synthetic brush, it will just melt).
Oil blacking is reasonably stable and doesn't require any additional sealing, although a layer of hard wax, reapplied every so often will make it a bit more durable.
I would recommend using linseed oil or similar rather than motor oil as motor oil (especially used) will contain all sorts of additives that you don't really want to be handling or inhaling. I've found that a mixture of linseed oil and paraffin wax works well.
Another option is to allow the piece to rust and then treat it with a rust converter such as Fertan, these are generally phosphoric and/or tannic acid based and will turn the rust a deep blue/black which can then be sealed with wax, oil etc
Depending upon how large and/long the chain is you may wish to use beeswax. Heat the chain to maybe a few hundred degrees (e.g., place in a container in an oven) then rub the wax on it. You can 'bake' it further to assure an even coat of wax.
On steel this gives a very nice black finish.
Equal parts beeswax and boiled linseed oil brush on clean, hot steel with a natural bristle brush is the standard for blacking. This creates a very durable finish for interior use. A brazed chain however suggests you've got brass and steel which will not finish evenly. Blacking with heat wont have the same effect on both metals. Your brass will shine through the finish.
For mixed metals especially if brass is involved I would use oil paints or if the closest approximation to traditional blacking is desired try lamp black with boiled linseed oil and brush it on cold then wipe the excess off.
Typically I would "blacken" a piece with an acetylene torch - covering the surface with pure carbon and the brush or wipe boiled linseed oil (which cures hard) over it. Wiping again over highlights to accentuate them by removing the carbon.
