Yesterday I prepared some 5/16 x 2" lag screws and some 1/4" x 1" carriage bolts for my dinner bell sets which have a swing arm bracket/hanger. To match the set they need a somewhat old time appearance. Recently purchased lag screws, machine screws, carriage bolts and other fasteners are commonly galvanized so the color doesn’t match when they are used with traditional black iron elements. I use this method to largely erase the "store-bought" look.
I remove the galvanizing by pickling in hydrochloric acid for aboit 10 minutes then rinse in water. I constructed a special 5 gallon bucket set up to do it. This was described in "The Pickling Bucket" post.
I also made a special Decorative Hardware Vise Tool which allows me to securely hold the threaded fastener in the vise and prevent damage to the threads while I do the finish work on the head. I used two pieces of 1.5" x 1.5" x 3/8" angle about 6" long to make the tool - see the diagram which shows my plan and the image which shows how I actually made it.
Any lettering or symbols on the head of the fastener are removed with a flap disc. The head is then heated with the torch and planished with a light hammer to produce a faceted surface. I try to avoid significantly deforming the perimeter of the heads so the proper wrench will still fit.
While hot, the surface can be brushed and wax applied to achieve the proper black color and protection.
Usually I plan projects to mount with 5/16" or greater diameter hardware but for short lengths I may use 1/4" diameter.
If I had a really big project to do I would probably just order the black, square head screws and bolts from one of the reproduction bolt companies or perhaps forge them from clipped head plow bolts which I can buy and the farm and ranch store.