The 'salt and vinegar' process features extensively on YouTube and is a simple and cheap option to try:
https://www.youtube.com/watch?v=frqjEzfIDVE
And:
https://www.youtube.com/watch?v=wp8CK9_PA1w
J.C. Penny - who supply an extensive range of products used by cabinet makers and antique restorers - sell 'Antiquing Fluid', which is a cold patination treatment which will colour new or bright brass, copper, bronze ,mild steel or nickel to give an antique look. It is ideal for use in antique restoration. The finish can be sealed with Rustins Jade Oil, wax polishes or lacquers. The brass & copper fluid is available in two colour tones - black and brown. The website states that this product is an alternative for the discontinued Liberon Tourmaline & Haematite products.
There are four types of fluid:
Black : For all copper based alloys & mild steel.
Brown : For Copper & Brass, on Gilding Metal and Silver.
Nickel Black : For nickel based metals.
Fedox : Acid solution for imparting a red oxide rust on ferrous metals, such as mld steel, cast iron etc.
The fluids will generally have little or no effect on stainless steel, aluminium or chrome.
In use, it's diluted 1:10, and not outlandishly expensive - 250ml for £8.34 inc VAT + P&P:
http://www.jpennyltd.co.uk/shopping/...tion.php?id=50
The website give instructions for use.
Hope that's of interest.