Oxalic acid -- otherwise known as dibasic acid, ethanedioic acid or acid of sugar -- occurs naturally in several plants as oxalates; in its refined form it is poisonous and extremely corrosive. The acid is easy to use and is one of the safer products for wood bleaching if used properly. While excellent at removing dark water stains from wood, it also removes any chemical stain previously used on the furniture.
Oxalic acid is highly corrosive and toxic in its refined form; use rubber or vinyl gloves whenever you handle it. Hazardous gases may cause health problems, so always mix and use it in a well-ventilated area. The crystal and sandpaper dust is potentially dangerous; use a dust mask and eye protection. Have a bucket of rinse water available to flush your eyes or to wash away spills if needed.
Oxalic acid crystals are available at most hardware shops; in addition, you need white vinegar, one or two large dry paintbrushes, two or more large clean sponges, a small plastic mixing bowl, a large plastic mixing bowl, a mixing stick, a plastic bucket of hot water, rinse water, rubber or vinyl gloves, goggles or other eye protection, a dust mask, a dust sheet, several clean rags, extra fine sandpaper (280-grit at least) and a tack cloth.
Begin by stripping old varnish, which may reveal stains. Use mild bleach on ink and chemical finish stains and oxalic acid for water stains, such as black spots and dark rings. Use dust sheets and sand away oil or remnants of varnish; sand in the direction of the grain. With a dry paintbrush, whisk away the dust, followed by a thorough rubdown with a tack cloth over the entire surface. Mix three parts oxalic acid crystals with one part hot water in a small plastic bowl to make a paste. Apply the paste to the stained areas with a brush or sponge, avoiding contact with any metal. Let this sit for three to five minutes for soft woods and for over an hour on hardwoods; if the crystals dry, either remoisten with a couple drops of water, or reapply. Once the desired colour is reached, remove the paste with a sponge.
Combine 30 ml (1 fl oz) of oxalic acid crystals with 250 ml (1 cup) of hot water in a larger plastic bowl to mix a "wash" and coat the entire surface to reach an even colour. When the item is consistently bleached, use a clean sponge and warm water to wash away all of the acid. To counteract the acid still saturating the wood, dilute ammonia with water 1:1 and use a clean sponge to wipe the entire surface. Immediately rinse again with clean water using a sponge and let the furniture dry for two days. Once the item is dry, use extra-fine sandpaper to gently sand the entire piece. Remove the dust with a clean, dry paintbrush, followed by a thorough rubdown with a tack cloth.