Through Lumholtz's efforts, Hartman gained experience in anthropological fieldwork and in the exhibition of anthropological materials. This period provided his first intensive exposure to the discipline of anthropology, including its subfields of ethnology, archaeology, and linguistics. By 1893, Hartman's research interests were shifting more and more toward anthropology.

Upon his return to Sweden in 1893, Hartman took a post at the Stockholm Botanical Garden, however his interest in anthropology continued. In 1894, he presented his anthropological paper, "The Indians of north-western Mexico," at the 10th International Congress of Americanists in Stockholm.

From 1896-1898, Hartman led his own anthropological expedition to Central America. Åke Sjögren, a Swedish mining engineer who had spent a number of years in Costa Rica, arranged and financed this expedition. This trip, nominally sponsored by the Swedish Society for Anthropology and Geography, involved research in Costa Rica, El Salvador, and Guatemala. Hartman conducted anthropological studies that included archaeology, ethnology, linguistics, and physical anthropology (anthropometry). At the close of the expedition, Hartman spent six months visiting the major museums in the United States.

Having established his credentials as an anthropologist, Hartman obtained his first curatorial appointment in a museum. Upon returning to Stockholm, he became the assistant to Hjalmar Stolpe, Director of the Ethnographical Section of the Naturhistoriska Riksmuseet (Museum of Natural History). Hartman's work in anthropology was greatly influenced by his mentor Stolpe, and he would later incorporate Stolpe's methods in his own field work. Hartman wrote his first major monograph about Costa Rica in 1901 entitled Archaeological Researches in Costa Rica.

In 1902, Hartman attended the 13th International Congress of Americanists in New York City. While in New York, he investigated employment opportunities at a number of museums in the United States.

In late January 1903, Hartman initiated correspondence with W.J. Holland, Director of Carnegie Museum, regarding employment. Within a month, Hartman was offered the position of Curator of the Section of Ethnology and Archaeology at the museum. On March 17, Hartman officially reported to work, and was very quickly dispatched on a seven-month expedition to Costa Rica later that month. Upon his return to Pittsburgh, Hartman attended a great number of professional meetings where he presented papers on his findings.

In 1905, Hjalmar Stolpe died, and Hartman returned to Sweden in 1908 to take the place of his mentor as Director of the Ethnographical Section of the Naturhistoriska Rikmuseet. Hartman's responsibilities and duties as a museum administrator restricted his research productivity, and he failed to publish much information about his archaeological excavations at various sites in Costa Rica. He held the position until 1923, when his declining health necessitated a medical leave of absence, from which he entered retirement in 1928. He died in Stockholm on June 19, 1941.

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