During the final three years of his life, French conceptual artist Yves Klein (1928-1962) created nine “empty zones” (invisible and intangible areas of empty space existing only in conceptual terms) which could only be purchased with pure gold. The buyer was then presented with a receipt, which could either be set on fire in a ritual between artist and owner, or preserve it, meaning that the immaterial purchase would be transferrable. One of these receipts is now coming up for sale at Sotheby’s, and its similarities to the modern-day NFT are so notable that Sotheby’s are going so far as to accept not pure gold, but cryptocurrency for it.