The Louisiana Purchase Exposition (LPE) was held in St. Louis, Missouri; it is often referred to as the St. Louis World's Fair. Originally, it was scheduled to open in the Spring of 1903 (during the actual centennial year of the Louisiana Purchase), but various construction - and other - delays pushed its opening into 1904. The Exposition ran from April 30 through December 1, 1904.
Over 19.5 million people attended the Exposition, which was held on a 1,200 acre site that, today, is occupied by St. Louis' Forest Park and Washington University. The Exposition hosted exhibits from 63 foreign nations, the US Government and 43 (of 45) states.
For more about the Exposition and the Gold Dollars issued to commemorate and support the Fair, see:
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1903/04 Louisiana Purchase ExpositionThe LPE was celebrated with many numismatic souvenirs. So many, in fact, that noted numismatist Robert Lynn Henershott (1898-2005) authored a catalog
The 1904 St. Louis World's Fair - The Louisiana Purchase Exposition: Mementos and Memorabilia for the LPE's material. The Hendershott catalog is considered the definitive work for the subject area.
The US Mint produced Award medals (popular among collectors of Exposition material) and souvenir medals for the Exposition. The souvenir medals, the focus here, were struck at the Exposition in silver (0.600 fine), two bronze versions (one of yellow brass alloy, one of a more traditional copper alloy) and gold-plated versions of each bronze piece. Exact mintage counts were not kept, but it is reported that approximately 90,000 total medals were sold during the LPE.
At the time of the Exposition, Farran Zerbe, the officially-appointed numismatic promoter for the Fair, reported that the majority of the medals struck/sold were a "brass-colored bronze" but that they proved to tarnish quickly and were replaced by a "copper colored medal." Zerbe also stated that a gold-plated version of each was available when the given base planchet was in use.
Per a notice in the January 1911 issue of
The Numismatist (the magazine of the American Numismatic Association), one gold medal was struck at the Exposition. It was struck for George H. Webber at the Mint exhibit in the US Government Building during the Exposition's operations. Mr. Webber provided gold dust from Alaska to be melted and used for the medal. Per HJ Slacker, the person in charge of the Exhibit, it was the only gold medal struck.
The silver version was first sold for $1.00, but later its price was increased to $1.25. The bronze versions were sold for $0.25 each and the gold-plated bronze versions for $0.50 each. Chas. (Charles) S. Muir secured the souvenir medal concession, and oversaw sales of the Mint medals at the Exposition.
1904 Louisiana Purchase Exposition Souvenir Medal - Silver, with Star Variety

The obverse of the medal features left-facing, conjoined portraits of US President Thomas Jefferson (front) and French Ruler Napoleon Bonaparte. The inscriptions "LOUISIANA • PURCHASE • EXPOSITION" and "OFFICIAL • SOUVENIR" encircle the portraits at the beaded rim.
On the reverse is seen a relief map of the continental US with the Louisiana Territory highlighted and identified. The St. Louis location of the Exposition is indicated by a star on the map, with rays emanating from it. (There are several minor varieties that feature different counts and lengths of the rays.) Above the map, at the rim, is the inscription "LOUISIANA TERRITORY" with "1803" "1,000,000 SQUARE MILES" and "$15,000,000" spread over three lines below it. Below the map is found the inscription "ST. LOUIS 1904" at the beaded rim.
George Morgan was responsible for the medal's design. He was a US Mint Engraver, at the time, working under Chief Engraver
Charles Barber (son of
William Barber, the previous Chief Engraver). He would ultimately go on to be appointed Chief Engraver of the US Mint in 1917 after Charles' death.
There is a noteworthy variety of the medal that does not include the star on the reverse. The "No Star" variety is more scarce than the "with Star" version.
1904 Louisiana Purchase Exposition Souvenir Medal - Gilt, No Star Variety
1904 Louisiana Purchase Exposition Souvenir Medal - Star vs. No Star Reverse

For other of my topics on commemorative coins and medals, see:
Commems Collection.