A key historical document from US history goes to auction at Heritage on May 21 with a $150,000 reserve.

The 1796 George Washington signed patent for a cotton gin has been charged by many with reinvigorating slavery in the US.

By the late 18^th century the use of slaves was on the wane in the US, with many slave owners finding the practice unprofitable.

The introduction of the cotton gin made the process of separating cotton seeds from fibers much faster and more cost-effective. This growth in cotton production saw an increased need for hands to pick the cotton on the plantations. Slavery was again a profitable undertaking.

The history changing patent has a $150,000 reserve

Although most people associate the cotton gin with Eli Whitney's invention of 1794, this document relates to a patent for Hodgen Holmes' improved version of two years later, which increased the efficiency of the process.

Washington's autograph is highly sought after among investors at auction, although rare gems can also be found on the private market.

Pete Daniel, a former curator at the Smithsonian's National Museum of American History, believes the patent is "a crucial document in analysing this important moment in history."

There were around 650,000 slaves in the southern states in 1790. That figure had risen to 1.3m by 1810.