Research showed that the porous stone may hold spore of bacteria that regrow under ideal conditions. So I put boiling water in the cup for about half an hour, changing the water when it cooled. Then washed the glass as usual. Thereafter I immersed the glass in butter milk for about four hours and again washed it clean.
The next day, I again poured about 50 ml of warm milk and waited for it to set. Though it took longer than usual, the milk did turn into curd and this time it tasted like our usual curd. Its texture, colour was normal.
Does this confirm claims that its fossil stone (from Jaisalmer) can curdle/set milk into dahi without a starter medium? These amateur trials at home certainly cannot prove anything but the fact remains that despite taking a longer time, my experiment did end with ‘dahi’ in the glass.
Dahi for thought?