Question: Who said Sharp sketched the house on the back of an envelope in 1973?
About the article "American Gothic", section "Creation".

Candidate passages:
In August 1930, Grant Wood, an American painter with European training, was driven around Eldon, Iowa by a young painter from Eldon, John Sharp, looking for inspiration.
Wood noticed the Dibble House, a small white house built in the Carpenter Gothic architectural style.
Sharp's brother suggested in 1973 that it was on this drive that Wood first sketched the house on the back of an envelope.
Wood's earliest biographer, Darrell Garwood, noted that Wood "thought it a form of borrowed pretentiousness, a structural absurdity, to put a Gothic-style window in such a flimsy frame house."
At the time, Wood classified it as one of the "cardboardy frame houses on Iowa farms" and considered it "very paintable".
After obtaining permission from the Jones family, the house's owners, Wood made a sketch the next day in oil on paperboard from the house's front yard.
This sketch displayed a steeper roof and a longer window with a more pronounced ogive than on the actual house, features which eventually adorned the final work.
Wood decided to paint the house along with "the kind of people I fancied should live in that house."
He recruited his sister Nan (1899–1990) to model the woman, dressing her in a colonial print apron mimicking 19th-century Americana.
The man is modeled on Wood's dentist, Dr. Byron McKeeby (1867–1950) from Cedar Rapids, Iowa.
The three-pronged hay fork is echoed in the stitching of the man's overalls, the Gothic window of the house, and the structure of the man's face.
However, Wood did not add figures to his sketch until he returned to his studio in Cedar Rapids.
He would not return to Eldon again before his death in 1942, although he did request a photograph of the home to complete his painting.
During the summer of 1930, Edward Rowan, a young gallery director from Cedar Rapids, a large city approximately to the northwest of Eldon, attempted to promote fine arts in the rural town by opening a gallery and library and leading art classes in Eldon.
Rowan's attempts were met with success—the "Eldon Forum" called the exhibitions "an unusual treat."
This, along with an indebtedness Wood felt toward Rowan, drew the painter (himself a native of Anamosa, Iowa) to come to Eldon.
In August, Wood was driven around the town by a young painter from Eldon, John Sharp, looking for inspiration.
Sharp's brother suggested in 1973 that it was on this drive that Wood first sketched the house on the back of an envelope.
Wood did not immediately regard the house as beautiful, but he did find it captivating.
His earliest biographer, Darrell Garwood, noted that Wood "thought it a form of borrowed pretentiousness, a structural absurdity, to put a Gothic-style window in such a flimsy frame house."
At the time, Wood classified it as one of the "cardboardy frame houses on Iowa farms" and considered it "very paintable."
After obtaining the permission of the Jones family, Wood made a sketch the next day in oil on paperboard from the house's front yard.
This sketch displayed a steeper roof and a longer window with a more pronounced ogive than the actual house, features which eventually adorned the final work; however, Wood did not add figures to the sketch until he returned to Cedar Rapids.
He would not return to Eldon again before his death in 1942, although he did request a photograph of the home to complete his painting. 
Arthur Elliot was an English artist who sketched dozens of lively pictures of Montreal, Canada, during his visit there from September 1881 to March 1882.
He had a sharp eye for the details of daily life and recorded them with an equally sharp wit.
The winter months, in particular, were of interest to Elliot, who recorded the various sleighs and the winter dress of Montréalers, and painted charming watercolours of skating carnivals and of children sledding on city streets. 
In December 2009 an envelope containing suspicious white powder was sent to Malik's office at the House of Commons, triggering an anti-terrorist investigation.
Comments written on the envelope suggested it had been sent by a supporter of the far-right.
The substance was later found to be harmless. 
The "thermal envelope", or heat flow control layer, is part of a building envelope but may be in a different location such as in a ceiling.
The difference can be illustrated by understanding that an insulated attic floor is the primary thermal control layer between the inside of the house and the exterior while the entire roof (from the surface of the shingles to the interior paint finish on the ceiling) comprises the building envelope.
Building envelope thermography involves using an infrared camera to view temperature anomalies on the interior and exterior surfaces of the structure.
Analysis of infrared images can be useful in identifying moisture issues from water intrusion, or internal condensation.