Drawing of the furnace of the stone wool insulation plant of Sofia Loka, Slovenija. Knauf. The steel Cupola melts the stone up to 1700 degrees. The fluid stone falls on the spinners, fast rotating wheels, thus forming fibers. Tekening van de steenwol oven in Sofia Loka, Slovenië. It was no pleasure making the drawing: fumes, wind, heat and ear splitting noise.