Marble is a natural calcareous stone, which means it is porous and susceptible to staining and discolouration. The best marble-whitening method is to use a combination of a mild cleaning agent mixed with an absorbent material known as "whiting" (finely ground white chalk), which can be found in most paint or home improvement stores. This paste mixture, called poultice, will then be applied to the marble surface and will act as a sponge, pulling out the stain.
For best results, poultice needs to be custom-mixed, to draw out the specific type of stain causing the marble's discolouration.
Put on gloves and mix the whiting powder with the cleaning agent in a bowl. If it's an oil stain, mix powder with baking soda and white spirit; for organic or biological stains, mix with 12 per cent hydrogen peroxide; for rust stains, mix with commercial rust remover; and for copper stains, mix with ammonia.
Blend the ingredients until a paste is formed that is as thick as peanut butter. The average 0.454kg. of poultice mix, will cover 1 square foot of counter top.
Wet the area to be whitened with distilled water. Spreading this thin layer over the counter top will fill the marble pores with water and immediately start to loosen the stain.
Spread a 1/4- to 1/2-inch thick layer of poultice to the counter top, using the wood or plastic scraper for even application.
Cover the entire area of poultice-covered counter top with the large sheet or sheets of plastic. With masking tape, seal the plastic by taping it down around the edges.
Remove the plastic after approximately 24 hours, but leave the poultice on the surface until dry to touch.
Remove the poultice by chipping with wooden or plastic scraper. Do not use a metal scraper; it may chip or scratch the marble.
Rinse the surface with distilled water and buff it dry with a soft cotton cloth.
Repeat the poultice application, if the counter top is still showing stain or discolouration. It may take several applications to remove deep colour damage.
There may be commercial options better suited for whitening marble. For example, copper stains are extremely difficult to remove, so hiring a professional may be a better option than using this do-it-yourself method.