This is a single-bath intensifier for underdeveloped films. It doesn't include some of the more hazardous and exotic components of other intensifiers. The silver nitrate of course needs to be handled with a lot of care.
Add to 500 ml water:
citric acid 1.5g
silver nitrate 2.5g
10 mins yields about a 2-grade increase. The solution does not keep, and turns milky after a while. I found this formula online somewhere, but can't find the source again.