When did the US government stop redeeming silver certificates?

Redemption of silver certificates for silver dollars was halted in March 1964 when the price of silver increased dramatically. The step was taken to prevent people from trading $1 silver certificates for coins that could be melted and sold for as much as $20 or $30 each.

After that date the government continued to redeem silver certificates for bullion, but the amount of metal that you could get depended on the daily price of silver.

By 1968 the precious metals market was fully privatized. The government ended all redemption of silver certificates for bullion on June 24 of that year because it was then possible to buy silver on the open market and there was no need for the Treasury to operate its own metal business.