Antique-looking screws and bolts are often needed to make reproduction 19th century furniture or other items that look authentic. Here's a cheap and easy way to make reproduction steel antique screws and bolts, by selecting the right modern ones and "antiquing" them.
Purchase modern steel screws or bolts with the typical bright, shiny plating. For the most common look of antiques from the mid to late 19th century, choose screws that have a regular slot head, not a phillips head, and choose bolts with a square head, not a hexagonal one, and square nuts also.
If the bolt heads have any raised markings, file them off. Antique screws made after the mid 19th century were generally machine-made and resembled modern ones. Screws made before the mid 19th century often were blunt on the ends and had off-center, hand-filed slots in the heads. You can imitate those by buying screws slightly longer than you need and cutting off the tips with a hacksaw or heavy wire cutters, and filing the slot slightly to widen it toward one side.
Finally, remove the bright metal plating. This can be done by putting the screws, bolts and nuts in a jar of vinegar for several days. At first, there won't appear to be any change, but after two or three days, the screws or bolts will begin to darken and in a few more days the plating will dissolve off them. Take them out and rinse them off, and they're ready to use.
Without a protective plating, your reproduction steel screws or bolts will rust if used outdoors without anything to protect them. You may like the rusty antique look, but if not, a period treatment such as rubbing the wood and the screws with linseed oil will help protect them from moisture.
After the vinegar has dissolved off the plating, discard it so that children or animals can't get to it, and don't use the jar for food products, since the metal plating is harmful if swallowed.