Borrowing from the Chrysler experience (Subaru head gasket problems)

There are now several new "Block Sealer" products out with sodium silicate in them, the most popular being the K&W brand. The alchemists called sodium silicate "water glass" since it turns to glass when dried. When added to coolant, first it is sucked into a crack by the intake suction, then the exhaust blast dries some of it out and turns it to glass, making the leak smaller. This continues with each stroke, the leak getting smaller and smaller as concentric layers of glass are laid down, until the leak stops leaking. Genuine MOPAR antifreeze, recommended in Chrysler owners manuals, contain a small amount of sodium silicate to stop internal leaks as they develop. Hence, leaking head gaskets and cracked heads are usually only a problem if you haven't been using genuine MOPAR antifreeze as Chrysler recommends. Sure you have them, but you never have cause to know about them.