Birkenstock London Black Oiled Leather Clogs For the London Clog, Birkenstock exclusively use the highest grade leather between 2.8 and 3 mm thick. The strap's adjustability ensures a perfect fit, making the clog a comfortable companion for many occasions. Narrow fit * Natural oiled leathers appear stressed and rustic looking. They have a visible grain and naturally occurring colour irregularities that are deliberately not removed in production in order that the natural matt surface of the leather is preserved. Applying a leather conditioner will minimise this appearance or rubbing scratches gently with your hand. The oily finish of all natural leather products results in Colour and Shade Variations making each and every pair unique.