Water gels, or slurries, were introduced in 1958. These were, at first, mixtures of ammonium nitrate, TNT, water, and gelatinizing agents, usually guar gum and a cross-linking agent such as borax. (Cross-linking is a form of chemical bonding.) Later, aluminum and other metallic fuels were sometimes used and vastly better gelatinizers were discovered. In addition nonexplosive sensitizers were developed that could replace the TNT if desired. When the highest possible concentration of strength is needed, however, large quantities of TNT are still used.
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