Blackening nuts & bolts ….

… and other small parts. Firstly, buy a kit of blacking solutions (details below) and some ‘Spirits of Salt’ (hydrochloric acid) from a hardware store. You’ll also need some plastic trays, a plastic funnel and plenty of wash water. And rubber gloves, eye protection and a face mask.

Start with a collection of tarnished zinc parts that aren’t very nice to work with. Zinced parts only!
Dissolve the zinc in dilute hydrochloric acid. It doesn’t need very much, maybe an egg-cupful of acid added to a small (plastic) dish of water. Do this OUTDOORS and WEAR A MASK, eye protection and gloves. When the effervescence stops, you don’t know if this is because all the zinc has been dissolved or the acid has ben used up. Add a tiny bit more acid. If it bubbles again, there is still some zinc left.
Rinse thoroughly with clean water and leave the parts under water to stop them rusting quickly. You’ll notice I had a stray non-zinc fishplate in this batch
Degrease the parts, stirring  for a few minutes, in the alkali provided in the kit. Rinse thoroughly.
Repeat with the ‘Activator’ solution, just for one minute. I use an old rod to stir with.:
After rinsing again in clean water, add the blacking solution itself and watch all the parts go black pretty quickly.
After a further rinse, drain off water and add the de-watering oil for a few minutes. This absorbs moisture from the parts.
Return the oil to the bottle, and use kitchen roll or similar to soak up as much of the oil as possible from the parts.
Leave parts in a tray in a warm place until they are dry. You can use them straight away difficult to phoograph all-black things – these are black!
A smaller batch shows them better
I found that the process didn’t work very well with plates and strips, the finish was too uneven. But for small parts the results are good. I prefer black fittings to nasty tarnished zinc ones

I got my kit from here: http://www.black-it.co.uk/ : about £40 including shipping for 500ml of each liquid. Sounds a lot but I processed over 2000 nuts and bolts, plus quite a few other parts. New nuts and bolts would have cost £200 or so!