The subjects celebrated by the classic-era US commemorative coin series range from seminal events in US history to minor local events that likely did not merit the issue of a legal tender US coin. Of all of them, one stands out for me in terms of disappointing sales. I refer to the 1926 American Independence Sesquicentennial Gold Quarter Eagle and Silver Half Dollar. Greater than 75% of the authorized (and struck) mintage for each went unsold and was returned to the Mint to be melted - this for coins that commemorated American Independence and the birth of the United States via the Declaration of Independence!
As I note at the start of each of these "Melting Pot" posts, I've written multiple times about the subject coin(s) - this time the American Independence Sesquicentennial coins - and prefer not to repeat too much of those previous posts here, so check out the links below for my previous posts on the topic coin(s) which provide more detail about the Sesquicentennial, the coins themselves and the involvement of the US Congress that brought about their creation.The coin program for the American Independence Sesquicentennial was a two-coin program: a Gold Quarter Eagle ($2.50) and a Silver Half Dollar. The Public Law that authorized the coins specified a limit of 200,000 Gold Quarter Eagles and 1,000,000 Silver Half Dollars; the sponsor/beneficiary of the coins was the Sesquicentennial Exhibition Association ("Association"). The Association was given the flexibillty, per the coins' authorizing legislation, to order its coins "in such numbers and at such times as they shall request" - no defined ordering period, no defined minimum orders!
The entire authorization of 1,000,000 Half Dollars (plus 528 coins for assay) was struck between May (514,274) and June (486,254) 1926; also, in May, the Mint struck 16,022 Gold Eagles, followed by a June mintage of 184,204 (226 assay coins). The Sesquicentennial Exposition opened in Philadelphia on May 31, 1926 and remained open through November 30, 1926. Considering these dates, it's clear that the Mint struck the coins in time for them to be available for sale at the Exposition during most of its run (in contrast to what happened with the 1920 Maine Statehood Centennial Half Dollar, for example).
The Association priced the Half Dollar at $1.00 per coin and the Gold Quarter Eagle at $4.00 each. Sales did not go as well as the Association had hoped - they could, in fact, be described as poor! This resulted in >75% of each denomination being returned to the Mint to be melted: 859,408 of the Half Dollar were returned/melted (85.9%), along with 154,207 (77.0%) of the Gold Quarter Eagles. It's hard to believe that coins marking the 150th anniversary of the nation's birth did not sell better! (Net mintage figures: Half Dollar: 141,120; Gold Quarter Eagle: 46,019.)
Trying to make sense of the sales results for the coins...
In the "Positives" column, the issue price for each coin was not outrageous vs. its face value or out of line with previous US commemorative coin programs, and the designs on each - while not remarkable - were certainly not bad, IMO. It would seem these attributes would support coin sales.
On the "Negatives" side of the ledger, the Exposition's poor attendance - partly due to it not being complete when opened on May 31st (which helped generate bad publicity for the Exposition which kept visitors away), partly due to the poor/rainy weather in Philadelphia that Summer - limited the potential of the on-site audience for coin sales.
All things considered, however, I would have expected better!
While it's true that ~141,000 Half Dollars sold along with ~46,000 Gold Quarter Eagles are decent-to-good sales totals within the context of the overall classic-era series - e.g., the Quarter Eagles sales total exceeded the total of all other classic-era gold commemorative coins and the Half Dollar sales total ranks within the Top 10 of all classic silver commemorative coin programs - the extremely high melt percentages point to the fact that a better sales volume was anticipated.
Though large numbers were melted, the net availability of each denomination in the marketplace appears adequate to meet current collector demand, with coins of each available at reasonable prices for all but true Gem condition (and above) coins.
1926 American Independence Sesquicentennial Half Dollar
1926 American Independence Sesquicentennial Gold Quarter Eagle

For other of my posts about commemorative coins and medals, including more on the American Independence Sesquicentennial coins, see:
Commems Collection.For a list of posts specifically about the Design Details of the 1926 American Independence Sesquicentennial, see:
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Design Discussions - 1926 American Independence Sesquicentennial - Half Dollar-
Design Discussions - 1926 American Independence Sesquicentennial - Gold Quarter EagleTo compare with the coin delivery experience of the 1920 Maine Statehood Centennial Half Dollar, see:
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1920 Maine Statehood Centennial - Late for the Show!