New York — What is Flow Blue china? Lots of people love it. Many collect it. But not many can describe exactly what it is. Flow Blue china was a popular dinnerware during the 19th century. Earlier, during the 18th century, as the Dutch and English traders brought blue and white porcelain china from the Orient, it was instantly sought after by the people of position and wealth in England.
Entrepreneurs soon designed shapes more to the English taste and made wooden models for the Chinese to reproduce in porcelain and then export back to England and Europe.
Domestic potters in and around Staffordshire began, meanwhile, to imitate these blue and white patterns on their much less expensive ceramic ware, and variations on the original Chinese themes became popular at middle-income levels.
In looking for less expensive ways than hand painting to apply the pattern to unfired pieces, potters found that fabric printed from an engraved copper plate could be pressed on the raw clay pieces before the glaze was applied.
During the firing, the pattern sometimes blurred the outlines. This was soon seen as an agreeable design ''plus,'' and ways were found to guarantee this ''flown'' look. Certain chemicals, in fact, would be added to the kilns during the baking process to ensure it.
A new style was thus born, and soon this blue and white tableware was sweeping both England and America. Hundreds of small potteries sprang up to turn out almost identical patterns, all of which were variations on the original Chinese.
For years, recent collectors of Flow Blue had plenty of difficulty in trying to identify which potters made their particular pieces. Until 1971 no comprehensive information was available. Petra Williams, author of the three definitive books on Flow Blue, speculates that transfer ware was considered a lowly subject for research, since most collectors were after the more desirable porcelain and historical ware made before the Industrial Revolution.
Mrs. Williams, who lives in Jeffersontown, Ky., inherited a few pieces of Flow Blue from her mother. She began to add pieces as she ran across them, and she wanted to know more about what she was collecting. Finding only casual references to the process of production and little, if any, descriptions of the designs themselves, she not only set out to do the research but wrote books on the subject.
The first is called ''Flow Blue China: An Aid to Identification,'' and was revised in 1981. The second is ''Flow Blue China II,'' and the third is ''Flow Blue China and Mulberry Ware - Similarities and Value Guide.'' All are published by Fountain House East, PO Box 99298, Jeffersontown, Ky. 40299, and cost $12.95 each, plus $3 for handling and postage. They are also available at the museum shop, or by mail, from the Strong Museum, One Manhattan Square, Rochester, N.Y. 14607.
Petra Williams built her collection of Flow Blue to well over 1,500 pieces. She has now donated her entire collection, which includes almost every known pattern of Flow Blue, to the Margaret Woodbury Strong Museum in Rochester, N.Y. It will be exhibited there until next January. This museum, opened to the public in October 1982, is unique in its collections of objects that were used or collected by Americans in the period spanned by the Industrial Revolution.
It includes fine art, household objects, books, toys, mechanical gadgets, advertising posters, fashions and furnishings that reflect the ingenuity and taste of 19th-century Americans. Named for Margaret Woodbury Strong, it houses the 300,000 objects she collected over her lifetime. The objects were all made in the period ranging from 1820 to the 1930s, an era of Americana that she considered much neglected.
The new $22 million museum, set in 13 acres in downtown Rochester, was built with a portion of the $60 million endowment she left to house and maintain her fabulous collection of Americana. It is open daily between 10 a.m. and 5 p.m., Tuesday through Saturday, and between 1 and 5 p.m. on Sundays. Admission is $2 for adults. Another current exhibition is called ''Inner Action: Automata and Mechanical Toys, 1850-1950,'' also on display until January. The museum also has one of the most extensive doll collections in the country.