Question: What is the year in which  Van Gogh wrote  details about the Portrait of Dr. Gachet to his brother?
About the article "Portrait of Dr. Gachet", section "Background".

Candidate passages:
The Gachet Family.
"Portrait of Dr. Gachet" is one of the most revered paintings by Dutch artist Vincent van Gogh of Dr. Paul Gachet, who took care of him in his last months.
In 1990, it fetched a record price of $82.5 million ($75 million, plus a 10 percent buyer's commission).
Gachet was also an artist of the Impressionist era.
He was an amateur painter and engraver.
Vincent van Gogh went to the doctor for medical care.
Van Gogh saw himself in the doctor; like himself, he saw in Dr. Gachet “the heart-broken expression of our time.” Similar to many of van Gogh’s portraits, the painting is a study not of the physical features of the man, but of the inner qualities of the doctor’s personality.
The portrait of Dr. Gachet notes all the tiniest details of his appearance and depicts them in the way that van Gogh sees them.
The characteristics of the doctor’s image are all freely painted so that the viewer may see them the same way that van Gogh does. 
Dr. Paul Gachet lived in Auvers-sur-Oise.
He was acquainted with the avant-garde artists of the time.
Through this connection, Vincent van Gogh moved to Auvers to be treated by him, though he considered the doctor to be in a worse state than himself.
Gachet befriended Van Gogh and was the subject of two portraits, one of which, "Portrait of Dr. Gachet", was sold at auction for over $80m (£48m) in 1990.
Van Gogh died by a gunshot to the chest.
The room on the upper floor of the Auberge Ravoux where he died has been preserved, although no furniture remains.
Auvers-sur-Oise is the final resting place of both Vincent and his brother Theo van Gogh, who died six months later.
In Vincent van Gogh's birthplace Zundert (NL), the street next to the small Protestant church where his father preached, has been renamed "Auvers-sur-oise-straat" (straat(NL)=street(EN)=rue(FR)). 
While Van Gogh was in Saint-Rémy, his brother, Theo and artist Camille Pissarro developed a plan for Van Gogh to go to Auvers-sur-Oise with a letter of introduction for Dr. Paul Gachet a homeopathic physician and art patron who lived in Auvers.
The plan also included him staying at a nearby inn while under the doctor's care.
Van Gogh had a room at the nearby Auberge Ravoux.
Van Gogh developed a close relationship with the doctor who he described as "something like another brother."
From the first meeting, though, Van Gogh sensed that Dr. Gachet may have been as ill as he.
In a letter from May 20, 1890, Van Gogh wrote, "I have seen Dr. Gachet, who made the impression on me of being rather eccentric, but his experience as a doctor must keep him balanced while fighting the nervous trouble from which he certainly seems to me to be suffering at least as seriously as I."
Van Gogh, introduced to etching by Gachet, made the etching "Portrait of Doctor Gachet" in 1890.
Gachet and Van Gogh discussed creating a series of southern France themes but that never happened.
This was the one and only etching, also known as "L'homme à la pipe" (Man with a pipe), that Van Gogh ever made.
Van Gogh's brother, Theo, who received an impression of the etching, called it "a true painter's etching.
No refinement in the execution, but a drawing on metal."
It is a different pose than that in Van Gogh's "Portrait of Dr. Gachet", owned by Musée d'Orsay.
The National Gallery of Canada finds that "The undulating flow of the line is typical of the expressive quality of Van Gogh's late style."
The impression owned by the National Gallery is from one of the 60 printings following Van Gogh's death by Dr. Gachet's son, Paul Gachet Jr. Gachet's collector's stamp appears on the bottom edge of the print. 
Van Gogh painted Gachet resting his right elbow on a red table, head in hand.
Two yellow books as well as the purple medicinal herb foxglove are displayed on the table.
The foxglove in the painting is a plant from which digitalis is extracted for the treatment of certain heart complaints, perhaps an attribute of Gachet as a doctor.
The doctor's "sensitive face", which Van Gogh famously wrote to Paul Gauguin carried "the heartbroken expression of our time", is described by Robert Wallace as the portrait's focus.
Wallace described the ultramarine blue coat of Gachet, set against a background of hills painted a lighter blue, as highlighting the "tired, pale features and transparent blue eyes that reflect the compassion and melancholy of the man."
Van Gogh himself claimed this expression of melancholy "would seem to look like a grimace to many who saw the canvas".
With the "Portrait of Dr. Gachet", Van Gogh sought to create a "modern portrait," which he wrote to his sister "impassions me most - much, much more than all the rest of my "métier"."
Elaborating on this quote, Van Gogh scholar Jan Hulsker noted "...much later generations experience it not only as psychologically striking, but also as a very unconventional and 'modern' portrait."
He also wrote, "My self-portrait is done in nearly the same way but the blue is the fine blue of the Midi, and the clothes are a light lilac," which would refer to one of his final self-portraits painted in September the year previous.
Van Gogh also wrote to Wilhelmina regarding the Portraits of Madame Ginoux he painted first in Arles in 1888 and again in February 1890 while at the hospital in Saint-Rémy.
The second set were styled after the portrait of the same figure by Gauguin, and Van Gogh described Gachet's enthusiasm upon viewing the version painted earlier that year, which the artist had carried with him to the home in Auvers.
Van Gogh subsequently carried compositional elements from this portrait to that of Dr. Gachet, including the table-top with two books and pose of the figure with head leaning on one hand.