Love letters from Frida Kahlo to Spanish artist José Bartoli will hit the auction block at Doyle New York on April 15. Kept secretly by Bartoli until his death in 1995, the letters offer new insight into Kahlo’s life and career.

Many of the letters, which total more 100 pages include keepsakes like photographs, drawings, and pressed flowers. They are estimated to bring between $80,000 and $120,000.

The pair met in New York while she was recovering from spinal surgery, and their affair continued after she returned to her beloved home in Mexico (see Photographs of Frida Kahlo’s House Show Paint and Blood).

In the letters, she writes of a pregnancy, her tumultuous relationship with her husband, Diego Rivera, and her artwork.

Kahlo specifically refers to the process of painting Tree of Hope, a 1946 double self-portrait in which one Frida lies on a hospital trolley with open surgical incisions, while the other, dressed in a red Tehuana costume, stares straight ahead.

Frida Kahlo, Tree of Hope (1946).

In several of the notes, she refers to Bartoli as her own “tree of hope,” writing: “I remembered your last words and I began to paint. I worked all morning and when I finished eating I kept on painting until there was no more light. But afterward I felt so tired and everything hurt.”

Kahlo biographer Hayden Herrera describes the unpublished letters as “steamy with unbridled sensuality.”

“Although Kahlo was deeply attached to Rivera, these letters suggest that she would have left him in order to live with Bartoli,” writes Herrera in an essay about the documents. “She told Bartoli that he gave her a kind of love that she had never experienced before. Her love for Bartoli was passionate, carnal, tender, and maternal.”

The auction comes during a time of renewed interest in the Mexican painter, as the Detroit Institute of Arts currently has a show of works by Kahlo and Rivera about their time in Detroit (see The Striking Absence in the Detroit Intitute of Arts’s Diego Rivera and Frida Kahlo Blockbuster, Madonna Refused to Loan a Frida Kahlo Painting to Detroit Institute of Arts for Blockbuster Show).