The bottle was designed to be interlocking, laid horizontally and bonded with cement mortar with a silicon additive. The necks were short and fitted into a large recess in the base, the bottles were square section with dimpled sides to ket to the mortar. A 10 ft (3.0 m) x 10 ft (3.0 m) shack would take approximately 1,000 bottles to build. In 1963, 100,000 WOBO’s were produced in two sizes, 350 and 500 mm. This size difference was necessary in order to bond the bottles when building a wall, in the same way as a half brick is necessary when building with bricks. Unfortunately, most of them are destroyed and no bottles are left. They are very rare and became a collector’s item.