â€œI had worked virtually every moment that I wasnâ€™t in school,â€ he wrote in a history for the company.
The firm begun in the 19th century by his great-grandfather Maurice got into numismatics as a sideline, buying and selling collector coins and currency in addition to its primary function in foreign exchange. It later diversified into dealing in antiques and rare stamps.
In 1935, after converting the company to a rare coin dealership, Morton and Joseph Stack held their first public auction. In 1953, Stackâ€™s moved to a gallery on 57th Street in Midtown Manhattan. (It is now on 38th Street and has galleries in other cities.)
In 2011, Stackâ€™s merged with Bowers & Merena to create Stackâ€™s Bowers Galleries.
Mr. Stack was the president of the Professional Numismatists Guild for two years beginning in 1989. In 1993 he was given the Founderâ€™s Award, the guildâ€™s highest honor.
In addition to his son, he is survived by his wife, Harriet (Spellman) Stack; his daughter, Susan; two grandchildren; and five great-grandchildren. He lived on Long Island.
The Stackâ€™s gallery was considered an inviting global clubhouse by many coin dealers and collectors. But Mr. Stack was not shy about promoting the companyâ€™s financial success.
â€œThere are people who sell gold and silver bullion, and rolls and bags of coins, who call themselves coin dealers, and some of them probably do upward of $100 million business a year,â€ he told The New York Times in 1984. â€œWhen you say â€˜rare coin dealers,â€™ though, and speak of firms that sell both directly and at auction, weâ€™re the largest coin dealer in the United States.â€
He drew a distinction between coin collectors, whom he courted assiduously, and investors.
â€œIf a collector and an investor had to abandon a sinking ship, the collector would take with him the rarest and most aesthetically appealing pieces without regard to market value,â€ he told The Times in 1977. â€œThe investor would try to take as much of his coins as possible, starting with the most valuable.â€