Question: Who did Van Gogh use as a model for his work?
About the article "Sien (Van Gogh series)", section "Abstract".

Candidate passages:
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). 
Sien (Van Gogh series)Vincent van Gogh drew and painted a series of works of his mistress Sien during their time together in the Netherlands.
In particular, his drawing "Sorrow" is widely acknowledged as a masterwork of draftsmanship, the culmination of a long and sometimes uncertain apprenticeship in learning his craft.
Commonly called Sien Hoornik, Clasina Maria Hoornik (1850 - 1904) lived with van Gogh during much of his time in The Hague from 1881 to 1883.
Van Gogh used Sien, a pregnant prostitute, as a model for his work and later took Sien and her daughter into his home.
Van Gogh made drawings and paintings of Sien and her daughter, baby and mother over that period, which reflected the domestic life and hardships of the working poor.
Their relationship was not accepted by his family or supporters, although his brother Theo did not withdraw his support over it.
It did contribute undoubtedly, however, to a split with Anton Mauve, a cousin-in-law and noted painter of the Hague School, who had introduced van Gogh to painting as well as supporting him financially, and whom van Gogh revered.
At his brother Theo's urging, van Gogh left Sien in 1883 to paint in Drenthe, putting an end to the only domestic relationship he was ever to have.
Sien resumed her life as a seamstress, cleaning woman and likely prostitute before marrying in 1901.
On 12 November 1904, aged 54, she threw herself into the Schelde river and drowned, fulfilling a prophecy she had made to van Gogh in 1883: "Yes, I'm a whore ... it’s bound to end up with me jumping into the water."
Vincent van Gogh's grandfather (born 1789) was also named Vincent van Gogh.
According to Vincent van Gogh's first biographer, his sister-in-law Johanna van Gogh, the grandfather was a pastor, and the son of Johanna van der Vin of Malines and Johannes van Gogh.
Johanna van Gogh writes that Johannes "was at first a gold-wire drawer like his father, but he later became a Bible teacher and a clerk in the Cloister Church at The Hague."
She describes him as an intellectual, duty-bound man who was awarded prizes and testimonials for his distinguished work.
A family legacy, from Vincent's great-uncle—a sculptor and a lifelong bachelor—allowed Vincent van Gogh (Johannes' son) to study divinity at the University of Leiden.
After successfully completing his studies and having become established at the parsonage of Benschop, Vincent van Gogh married E. H. Vrydag in 1810.
They remained married until Elisabeth's death on 7 March 1857; the Reverend Vincent van Gogh lived until 1874.
In July, 1881 Van Gogh made "Portrait of Artist's Grandfather" (F876).
The work was drawn in pencil and brown ink.
Then he used opaque white watercolor and a brown wash.
The drawing is owned by the Van Gogh Museum in Amsterdam. 
"Vase with Zinnias and Geraniums" (F241) reflects the influence of Adolphe Monticelli (1824–1886) in its vivid color and impasto paint.
Van Gogh admired, and later collected, Monticelli's work.
Van Gogh had been introduced by his brother Theo to Monticelli's still life work with flowers in Paris.
He admired Monticelli's use of color as an expansion of Delacroix's theories of color and contrast.
Secondly he admired the effect Monticelli created by heavy application of paint called "impasto".
It was partially Monticelli, from Marseilles, who inspired Van Gogh's southerly move to Provence in 1888.
He felt such kinship for the man, and desire to emulate his style, that he wrote in a letter to his sister Wil that he felt as if he were "Monticelli's son or his brother."
The first owner in the provenance of this painting is "C.M.
van Gogh Gallery, Amsterdam, The Netherlands", which was owned by Van Gogh's uncle and an art dealer, Cornelius Marinus van Gogh (1824–1908).
"Bowl with Zinnias and Other Flowers" (F251), painted within days of "Vase with Zinnias and Geraniums" (F241), is evidence of Van Gogh's transition to a lighter color palette.
Picking up elements of Impressionism Van Gogh painted with a more vigorous brushstroke, with thick application of paint, called "impasto" which created a three-dimensional relief.< 
In 1880 when he was 27 years-old, Van Gogh decided to become an artist.
In October of that year he moved to Brussels and began a beginner's course of study.
Van Gogh returned to Etten in April 1881 to live with his parents and studied art on his own.
His younger brother Theo, an art dealer at the main branch of Goupil & Cie, encouraged him and started covering Van Gogh's expenses.
Van Gogh used images from illustrated magazines to teach himself how to draw.
Charles Bargue, a French artist, wrote two books on drawing that were a significant source of study for Van Gogh.
Both written in 1871, one is "Cours de dessin" and the other "Exercises au fusain pour préparer à l’étude de l’académie d’après nature," Van Gogh created copies of the drawings or adapted nude images for clothed depictions.
In January 1882, he settled in The Hague where he called on his cousin-in-law, the painter Anton Mauve (1838–88).
Mauve introduced him to painting in both oil and watercolor and lent him money to set up a studio.
Van Gogh began drawing people from the working poor, including the prostitute Clasina Maria "Sien" Hoornik (1850–1904, with whom he would become involved.
Van Gogh dreamed his studio would one day become a form of respite for the poor where they could receive food, shelter and money for posing.
His work was not well received; Mauve and H.G.
Tersteeg, manager of Goupil & Cie, considered the drawings coarse and lacking charm.
Van Gogh likened the subjects "unrefined" characteristics of his drawings to harsh "yellow soap" made of lye.
Van Gogh's relationship with Sien changed as they lived together with Maria, her five-year-old daughter, to the great disappointment of his family.
Mauve appears to have suddenly gone cold towards Van Gogh and did not return a number of his letters, Van Gogh supposed that Mauve did not approve of his domestic arrangement with Sien and her young daughter.
Van Gogh created a number of drawings of Sien and her daughter.
Having ended his relationship with Sien, Van Gogh moved to Drenthe in September 1883 and painted landscapes.
Three months later, he moved back in with his parents, who were at that point living in Nuenen.
In 1884, Van Gogh created the weaver series, works of rural life and landscapes.
For a brief period of time, he gave painting lessons in Eindhoven.
Near the end of the year, Van Gogh began experimenting with complementary colors, influenced by the color theories of Charles Blanc.
Vincent made many studies of peasants in 1885, culminating in his first major painting, "The Potato Eaters."
In his works, Van Gogh used particularly somber colors and colors mixed with black, which he felt was like that of 17th-century masters, such as Frans Hals.
His brother, Theo van Gogh (art dealer) had asked him often to lighten up his work, referring to the work of the Impressionists.
Once Van Gogh went to Paris, he did open up his palette to color and light and admitted that his works from this period were old-fashioned.
Theodorus van Gogh, Vincent's father, died March 26, 1885.
In November, Vincent moved to Antwerp.