But a peculiar falling- out occurred. The museum needed to move, and its leaders ultimately selected a site at the edge of town, infuriating Horst, who demanded the return of his art and money.
The museum complied, and in 1926 the indignant magnate built a large building in the woods near his home to house and show his paintings and sculptures.
And there they stayed.
Now, 88 years later and 80 years after Horst's death, the intact collection is going to auction March 30 at Freeman's auction house in Philadelphia. Visitors to Freeman's Wayne outpost can view highlights of the sale through Wednesday; there will be an exhibition at Freeman's Chestnut Street gallery March 26 to 29.
"The paintings represent a slice of the American art market at a particular moment," said Scott Schweigert, the Reading museum's curator of art and civilization. "The Horst collection speaks to what was shown in Boston, New York, Philadelphia. Painters, the best painters, all wanted to be exhibited at these shows. For Horst, it was a way for him . . . to have his finger on the pulse of what was happening in American art at the moment."
The top paintings have pre-auction estimates of $200,000 to $300,000 (works by Childe Hassam, Daniel Garber, William Redfield, and Frank Weston Benson). Most of the collection, however, consists of works estimated to sell for less than $10,000, often much less.
George Sullivan, Horst's grandson, said that after Horst died, the gallery building was used for entertaining (Horst himself hosted many an evening there, too), but the building was largely closed.
"It was used occasionally for parties, and that was it," said Sullivan, 67. "Up there it sat. All those years, gathering dust."
Well, not entirely. While Freeman's is marketing the collection as "undisturbed . . . since 1929," the paintings are not unknown to scholars and curators, and the family has lent works to various institutions.
In fact, in 1996, a large number, about 40 canvases, were on display at the Reading museum as part of a big show, "Reading Collects." Hassam's The Norwegian Cottage (1909), part of the Freeman's auction, served as the postcard image advertising the show.
Even the Pennsylvania Academy, source of many Horst acquisitions, has exhibited works on loan. "Daniel Garber: Romantic Realist," a 2007 exhibition at the academy and the Michener Art Museum, featured a Horst-collection Garber, Glen Cuttalossa (1925); now it's going to auction.
Still, it is a bit extraordinary to come upon such works in an isolated building on woody farmland south of Reading.
"I've been to the property, I've seen it in situ, I was lucky enough to see it there," Schweigert said of the museum. "It is this sort of time capsule. . . . I can imagine someone walking in there, in the middle of the woods, and saying, 'What?' In the context, you don't expect it."
Apparently, Horst himself did not expect it either, but his pique got the better of him.
The Reading museum, first housed in the school district administration building downtown, quickly grew, in no small measure because of Horst's money and art. By the early 1920s, the place was full and it was clear a new building was necessary.
Horst believed a museum should be built in the city's central park - to serve as a visible emblem of Reading's cultural heft accessible to all.
Other influential civic and museum leaders believed another site was more appropriate. In 1924, while Horst was vacationing in Europe, museum leaders agreed to accept a donation of land at the far edge of town from Horst's principal business competitor.
Horst's response was quick and sharp.
He referred to the site as "swampland." He noted it was practically unreachable. He described the area as an "unsightly and ill-smelling ash dump."
"I will of course refuse to let any of my paintings go out of the city and shall make provisions to have them exhibited at some other more suitable place," he wrote the museum's founding director, Levi W. Mengel.
He then asked for the return of his art and his money. The museum returned 39 paintings, retaining 20 that had been given permanently.
Horst then built his gallery - outside town - and used it primarily for entertaining.
George Sullivan's parents also hosted dinners there.
But now Sullivan and his three sisters are selling much of the family land, including the site of the gallery, and the paintings are coming to market.
"There's an irony in Horst's saying [the art] will never leave the city," said the museum's Schweigert.
"It's sad in a way," Sullivan said. "Time marches on. We're getting old."