Once relegated to the relative obscurity of technical jargon, the words "sodium silicate" (a.k.a. liquid glass) have gained some notoriety of late. While sodium silicate is now well known as the one-size-fits-all destroyer of engines under the government's "cash for clunkers" program, it was once primarily used for an entirely different purpose: To save them.
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Sodium silicate is a metal salt in the same family as sodium carbonate (commonly called "washing soda," used as a substitute for lye and as a chemical water softener). Sodium Silicate (chemical name Na2SiO3) is the result of combining sodium carbonate and silicon dioxide (the primary ingredient in sand).
In its dry form, sodium silicate is a yellowish powder resembling table salt. Like table salt, sodium silicate easily dissolves in water and remains as a residue when the water evaporates away. However, unlike salt, sodium silicate molecules bond together into a solid sheet of glass (silicon dioxide) when dried instead of reverting to a powder.
As a Sealant
Blown head gaskets, engine block and cylinder head cracks tend to occur in very hot areas. Such cracks usually aren't very wide (usually less than a millimetre), but are often just wide enough for water to steam through or hot gasses to enter the cooling system. A certain amount of the coolant flowing through the crack gets converted into steam upon exiting the system, leaving behind molecules of liquid glass in the crack. The extreme heat inside that crack melts the liquid glass molecules, helping any subsequent molecules stick together and form a solid sheet of glass.
Liquid glass doesn't expand or contract at the same rate as steel or aluminium, meaning that subsequent overheating can fracture and/or dislodge the glass repair. This can even happen in the course of normal usage if the area in question happens to get a little hotter than surrounding areas. In theory, there is no limit on how wide of a crack liquid glass may fill, but larger cracks are more prone to fail later.
Sodium silicate is very easy to use; you simply dump it into the radiator and run the car for 15 to 20 minutes or however long it takes to seal the crack (whichever comes first). You must thoroughly flush the cooling system immediately after treatment to keep the liquid glass from building up where it shouldn't. Although many block repair products bill themselves as permanent, the reality is that liquid glass repairs often last between two and four years without reapplication of sodium silicate to repair cracks in the patch.
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