French chalk is a type of talc used by tailors for marking cloth, by cleaners for removing grease from cloth and as a dry lubricant in its powdered form.

French chalk is also an essential ingredient required for the repair of punctured inner tubes of pneumatic tires, such as are found on bicycles. A block of French chalk is typically included in proprietary puncture repair kits. These kits typically include some means of generating a fine powder from the block, for example a small file or a roughened area on the metal case in which the repair kit may have been supplied.

To repair a puncture, the relatively fragile inner tube is removed from its normal location inside the tire. The puncture is identified by inflation of the inner tube whilst it is held under water. The site of the puncture is identified by a stream of bubbles. After marking the site of the puncture and drying the inner tube, a patch is secured over the puncture using a suitable adhesive. A fine dusting of French chalk is then applied to the same region to prevent adhesion of the inner tube to the inner surface of the tire after it has been replaced and inflated to the correct pressure.

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Source: adapted by the editor from Wikipedia, the free encyclopedia; from the article "French chalk". Image Credit.

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