[OLDIES BUT GOODIES]
A very pure, renowned French still spring-water as a colour contrast penetrant.
In the late '70s/beginning '80s in France, the nuclear industry was at full speed: up to six power plants commissioned every year!
In October '78 two very important companies involved in this area issued a new specification dealing with penetrant testing. This specification required, among other points, that the materials' fluorine + chlorine content on one hand and sulphur content on the other hand be 200 ppm (parts per million or 0.02 %) by weight maximum when analysed as per a specific method - not the "evaporation and extract analysis" one, as described in the ASME code.