Gradierwerk Bad Dürrenberg, Graduation tower in Bad Dürrenberg, Germany.

Gradierwerk Bad Dürrenberg is a wooden graduation tower in Bad Dürrenberg, Germany, built to concentrate brine by letting it trickle through bundles of blackthorn branches. The structure stands several meters tall and runs for a considerable length, with the wood framework open on the sides to allow air to pass through freely. It is the longest continuous graduation tower in Germany, with 636 meters of the original 1887 meters still preserved.

Johann Gottfried Borlach began geological surveys around Dürrenberg on 25 June 1741, and shaft sinking started on 3 May 1744. After 19 years of work, miners struck the sought brine at a depth of 223 metres on 15 September 1763; four days later construction of the first graduation tower began, and on 16 March 1765 the first 100 hundredweight (5000 kg) of boiled salt were produced. Medical use of the brine began in 1846, salt production closed in 1963, and the spa operations ended in 1965.

People come to walk along the structure not just to look at it, but because the salty air near it is noticeable and refreshing. The fine spray that forms as brine drips through the wood creates a light mist you can feel on your skin.

The structure sits in an open area in Bad Dürrenberg and can be visited throughout the year, though walking near it on a breezy day gives a better sense of how it works. A path runs along its length, making it easy to walk the full stretch and see the wood framework up close.

The blackthorn branches inside the structure are not permanent and need to be replaced over time because the constant flow of brine gradually breaks down the wood. This means the framework you see today is not the original material but a regularly renewed version of it.