It's caused by the rubbing together of the hinge parts. Microscopic pieces of the metallic coating plus dirt and lubricating oil combine to produce that nasty residue that eventually falls to the floor. Here's how to minimize it. First, remove the hinge pins and then the door from its opening.
ANSWER: That black sooty stuff that seems to come out of the door hinges is graphite.
Dust caused by hinges is common with residential duty hinges and is caused by the pieces of metal rubbing against each other. From what I hear a little lubrication with lithium grease can help though I don't think it will eliminate the problem.
Once the hinge pins are removed, you can clean the hinges. You can use a clean cloth and a small brush to remove any dirt or debris from the hinges. You can also use a solution of warm water and mild detergent to clean the hinges. Dip the brush in the solution and gently scrub the hinges until they are clean.
If you have chrome hinges, you can clean them with white vinegar. Mix equal parts of vinegar and water and scrub the hinges with a brush. Rinse with clean water and then dry with a soft cloth. Polish to shine.
You're going to make a paste which will clean the tarnish away for you. Take a bowl and measure in one tablespoon of flour, one of salt, and one of vinegar. Mix it thoroughly until it turns to a paste, and then rub it on to the surface of the brass or copper.
For especially dirty hinges, also add 1 tablespoon (15 ml) of white distilled vinegar and 1 tablespoon (12 g) of baking soda to the mix. Don't let hinges soak in vinegar for too long or else they could start corroding.
Take the door hinge, recently scrubbed, and place it in a container filled with some form of cola beverage. These beverages contain phosphoric and carbonic acids which remove rust from surfaces. Live the hinge in there for several hours or even up to a day, checking periodically for the etching progress.
Do-it-yourself-ers commonly use WD-40 on squeaky door hinges, but Brian Gow, President of Scheel Window & Door, says that the product attracts dust, which won't help the issue. “As a result, your door hinges will slowly darken and turn black and dirty looking,” Gow says.
If you are experiencing the same issue as Evelyn, you need to know that the black stuff that is forming on your door is mold. It is common to find the exterior side of doors that have been painted with an oil-based paint have mold growing on them.
Stubborn stains
There's no need to panic though. Simply pop on some rubber gloves and apply a few drops of paint thinner (also known as liquid spirits) to a clean rag and then rub with the grain of the wood to remove stubborn stains or seriously built-up dirt and grime. This should do the trick.
For door hinge oil, your best option is to use silicone spray or white lithium grease. Silicone is the easiest, but it does not last as long as the grease.
Painting hinges is a tricky process, fraught with potential application errors. Most amateur painters don't know that metal is ill-suited for paint adhesion. If you apply paint directly over under-prepared hinges, expect to see rampant flaking.
This tends to give the door hinge (considering a variety of factors) an average lifetime of around 10-15 years. This however, does not mean that you must replace them immediately. In fact, provided they are maintained well and used in an orderly fashion then it could be longer.
Baking Soda (or Vinegar) & Heat
You can do this on your stovetop with a disposable pot or pan. For every quart of water, add 1/4 cup of baking soda or vinegar and bring the water to a boil. Add the item to the pan and let it boil for about 15 minutes or until the paint falls off.
Regardless of whether you wish to lubricate your door hinges or to lubricate something else, the fact of the matter is that WD-40 can be used effectively for the job. The WD-40 multi-use-product spray can not only be used as an effective door lubricant but for many other purposes as well.
If you're looking to save money on restoring your steel, common household items that help with tarnish removal include: Olive oil. Vinegar. Baking soda.
The best option is to use a commercial copper or brass cleaner that is mildly abrasive. Apply a thin layer, allow to dry, and then buff. You might need to repeat the process if your hardware is very discolored and dirty.