Anandakanda 1.23.217

1.23.217

ruddhvā dvādaśayāmāntaṃ vālukāyantrake pacet /
sphoṭayet svāṃgaśītaṃ tamūrdhvasthaṃ gandhakaṃ tyajet //

Having sealed it, one should cook it in a sand apparatus up to twelve watches;
one should break it when cool of its own accord [and] discard the sulfur residing above.