Question: Who was Van Gogh writing when he wrote: "It is a kind of painting that rather changes in character, and takes on a richness the longer you look at it."?
About the article "Sunflowers (Van Gogh series)", section "Sunflowers, friendship and gratitude".

Candidate passages:
Van Gogh began painting in late summer 1888 and continued into the following year.
One went to decorate his friend Paul Gauguin's bedroom.
The paintings show sunflowers in all stages of life, from full bloom to withering.
The paintings were considered innovative for their use of the yellow spectrum, partly because newly invented pigments made new colours possible.
In a letter to his brother Theo, Van Gogh wrote: "It is a kind of painting that rather changes in character, and takes on a richness the longer you look at it.
Besides, you know, Gauguin likes them extraordinarily.
He said to me among other things - 'That...it's...the flower.'
You know that the peony is Jeannin's, the hollyhock belongs to Quost, but the sunflower is somewhat my own."
The works.1882.
On a blustery day, Van Gogh set up his easel and painted "plein-air" (in the open air) at a beach resort, Scheveningen, near The Hague to paint "View of the Sea at Scheveningen" (F4).
While Impressionists are often given credit for painting outdoors, they were not the first to do so.
Most, however, made sketches on the spot and worked on the painting in a studio.
In this case, Van Gogh struggled with the strong wind which sent grains of sand into his thickly applied paint.
Although most of the sand was scraped off, there are still a few grains of sand enmeshed in the layers of paint.
The tumultuous weather is well depicted with white-capped seas, threatening sky and wind-blown flags.
This painting was stolen from the Van Gogh Museum on December 7, 2002 and, at the time of this article, is no longer on display.
Of a study that Van Gogh made for "Girl in a Wood" or "Girl in White in the Woods," (F8) he remarked at how much he enjoyed the work and explains how he wishes to trigger the audience's senses and how they may experience the painting: "The other study in the wood is of some large green beech trunks on a stretch of ground covered with dry sticks, and the little figure of a girl in white.
There was the great difficulty of keeping it clear, and of getting space between the trunks standing at different distances - and the place and relative bulk of those trunks change with the perspective - to make it so that one can breathe and walk around in it, and to make you smell the fragrance of the wood.
"In "The Girl in the Woods" (F8a) the girl is overshadowed by the immense oak trees.
The painting may be reminiscent for Van Gogh of the times in his youth he fled to the Zundert Woods to escape from his family.
In November 1882 Van Gogh began drawings of individuals to depict a range of character types from the working class, "Worn Out" was one of the series.
The works were drawn in a black in an angular style.
On November 24, 1882 Van Gogh wrote to his brother Theo of Adrianus Zuyderland, the man who posed for "Worn Out" (F997), "What a fine sight an old working man makes, in his patched bombazine suit with his bald head".
Zuyderland was resident of the Dutch Protestant Almshouse for Old Men and Women.
Van Gogh would offer a small payment for residents who would pose for him.
On his pencil drawings, Van Gogh used milk as a fixative to counteract the graphite pencil shine and left the image "velvety black".
While it was recommended to use an atomizer to control the amount of milk placed on the work, Van Gogh would pour an entire glass of milk on the work.
In this drawing there is a noticeable stain around the drawing where the puddle of milk dried.
"Worn Out" is not displayed on this page, but can be found on the Van Gogh Museum website.
Unlike the character studies Van Gogh wrote to his brother Theo in November 1882 that he had drawn a portrait of Jozef Blok (F993), a street bookseller who was sometimes called "Binnenhof's outdoor librarian".
The work was detailed in pencil with watercolor and chalk.
At this time it was rare for Van Gogh to use color, as he found it difficult to work with. 
During March and the beginning of April 1885 he sketched studies for the painting, and corresponded with his brother Theo, who was not impressed with his current work or the sketches Van Gogh sent him in Paris.
He worked on the painting from April 13 until the beginning of May, when it was mostly done except for minor changes which he made with a small brush later the same year.
Van Gogh said he wanted to depict peasants as they really were.
He deliberately chose coarse and ugly models, thinking that they would be natural and unspoiled in his finished work: "You see, I really have wanted to make it so that people get the idea that these folk, who are eating their potatoes by the light of their little lamp, have tilled the earth themselves with these hands they are putting in the dish, and so it speaks of manual labor and — that they have thus honestly earned their food.
I wanted it to give the idea of a wholly different way of life from ours — civilized people.
So I certainly don’t want everyone just to admire it or approve of it without knowing why."
Writing to his sister Willemina two years later in Paris, Van Gogh still considered "The Potato Eaters" his most successful painting: "What I think about my own work is that the painting of the peasants eating potatoes that I did in Nuenen is after all the best thing I did".
However, the work was criticized by his friend Anthon van Rappard soon after it was painted.
This was a blow to van Gogh's confidence as an emerging artist, and he wrote back to his friend, "you...had no right to condemn my work in the way you did" (July 1885), and later, "I am always doing "what I can't do yet" in order to learn how to do it."
(August 1885). 
Van Gogh wrote to his brother Theo in November 1882 that he had drawn a portrait of Jozef Blok (F993), a street bookseller who was sometimes called "Binnenhof's outdoor librarian".
Unlike the character studies, the work was detailed in pencil with watercolor and chalk.
At this time it was rare for Van Gogh to use color, as he found it difficult to work with (see early works of Vincent van Gogh). 
The "One-eyed Man" is a portrait of a patient at the Saint-Paul asylum in Saint-Rémy.
Van Gogh used sweeping brushstrokes in the painting made with green in the background, the man's coat and touches in his face.
In the hospital Van Gogh found that in the presence of other patients he became more calm about his mental state.
In a letter to his mother he wrote, "At the moment I am working on a portrait of one of the patients here.
It is odd that if one is with them for a time, and has grown used to them, one no longer thinks them mad.
"There's a problem with timing here.
Van Gogh was not in Saint-Paul's until 1889.
Maybe the patient that Van Gogh referred to in the letter to his mother was about Portrait of a Patient.