The blue and black colouring are wood defects, which occur in the sawmill or at the wood merchant during artificial drying, caused by a fungus, which grows on the wood and leaves discolorations, which look like ink stains, the black colouring is caused by iron particles of nails, screws, saw blades or similar, which react with the tannic acid contained in the wood, especially oak wood in this way.

They need:

Oxalic acid.

Rubber gloves, goggles and overalls.

.container made of glass, ceramic, plastic or enamel

.wooden spatula

Synthetic brush.

Sandpaper and dust mask.

Cotton cloth.


Oxalic acid is used to bleach black stains and can also be used to remove dirt from old wood, especially oak, or to degrease oily surfaces such as teak.



1. oxalic acid is available as a white powder in crystal form. the acid is a toxic substance, so wear gloves, goggles and an overall and wash your hands very thoroughly after work. use glass, plastic or ceramic containers and a wooden spatula to mix and store the powder. prepare a saturated acid solution by adding the powder to warm water and stirring.

2.add powder until the crystals do not dissolve.about a cup of solution is needed to bleach a tabletop.always add the crystals to the water and not vice versa.leave the solution to stand for about 10 minutes before use.

Apply the oxalic acid solution with a synthetic brush and the hair of a normal brush would dissolve.
Brush the bleach evenly over the stain without splashing, wearing protective glasses.

4. leave the solution in the wood for about 30 minutes, stubborn stains may need repeated treatment. Use a clean cloth and clear water to rinse the treated area.

5. It is important that you rinse well with plenty of water to remove even the last traces of acid.

If you are sure that there are no acid residues, allow the workpiece to dry thoroughly before sanding and further treatment, using a dust mask.

Occupational safety and health

Always wear rubber gloves, goggles and protective clothing.   . 
Only work with bending agents outdoors or in well-ventilated rooms.
Follow the manufacturer's instructions exactly,
If you get bleach on your skin, wash it off immediately with plenty of water.
Work within reach of a drain basin. Ä Z.
Wear a dust mask when sanding the surface after bleaching,

Store Chemikaiien in a cool, dark place.

Always keep chemicals away from children and never fill them in food containers - children may accidentally eat or drink them!



When machining a larger surface, such as a table top, clamp a crossbar under the top so that it cannot warp.

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