On November 28, 1961, President Kennedy halted sales of silver by the Treasury Department. Increasing demand of silver as an industrial metal had led to an increase in the market price of silver above the United States government's fixed price. This led to a decline in the government's excess silver reserves by over 80% during 1961. President Kennedy also called upon Congress to phase out silver certificates in favor of Federal Reserve notes.Wikipedia: Executive_Order_11110
President Kennedy repeated his calls for Congress to act on several occasions, including his 1963 Economic Report, where he wrote:
I again urge a revision in our silver policy to reflect the status of silver as a metal for which there is an expanding industrial demand. Except for its use in coins, silver serves no useful monetary function.
In 1961, at my direction, sales of silver were suspended by the Secretary of the Treasury. As further steps, I recommend repeal of those Acts that oblige the Treasury to support the price of silver; and repeal of the special 50-percent tax on transfers of interest in silver and authorization for the Federal Reserve System to issue notes in denominations of $1, so as to make possible the gradual withdrawal of silver certificates from circulation and the use of the silver thus released for coinage purposes. I urge the Congress to take prompt action on these recommended changes.
So wie ich es verstehe wollte er einfach Silber aus dem Geldsystem ziehen, da es zu wertvoll war. Vor dem Kongress und nicht bei Privatleuten.