It only works with lead/tin/antimony alloys. Lead and tin alloy will not harden by water dropping but it will age harden over about two weeks a couple of BHN points!
With lead and tin alloys you max out at about 10-12 Brinnel after about 14 days but with oven heat treated lead/tin/ antimony in 90/5/5 ratio with water drop, you can reach 40 BHN and yes they are brittle.
Also lead/tin alloys begin to soften after about a year unless kept in a freezer which will maintain hardness longer.
Antimony alloys do not exhibit this spontaneous softening tendency.
I cast some wheel weights over 15 years ago with a water drop from the mold and they still are within a BHN point or two of when cast.
I'm told these are about a 92/4/4 ratio.