Much of this is true of antiques, as well, but there is no similar index of their values, according to Moses. Because most antiques sales are conducted through private dealers, he explained, it's too hard to find public sources of concentrated data for meaningful price comparisons over time.
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As any viewer of TV's "Antiques Roadshow" knows, the value of antiques can rise over time, as specific craftsmen, styles or periods become popular.
But while an antique individual dresser or chest might soar in value, a virtually identical one might not, because someone refinished it or changed the hardware. Because each antique or work of art is unique, prices fluctuate wildly. This creates both opportunities and hazards for investors.
"Art, like real estate, is a heterogeneous good," Moses said. "Every object is different."
"You can get two people who really want something and they will pay an astronomical price," he said. "And then you get someone who really needs to unload something and there's only one buyer."
Both experts agreed that investors in antiques or art need to start with an appreciation of the objects, not a desperate need to make money.
People who become investors—actively seeking works for their potential returns—almost always start out as collectors simply buying works they admire.
"I'll give them the pros and cons," said Yardumian, describing his approach with clients interested in art. "I'll talk to them about the nature of investing in art, [which is] significantly different … than investing in real estate, stocks or bonds or mutual funds."
Art and antiques do not provide the steady income one might earn from stock dividends, bond coupons or rent on property, he noted. So money put into art and antiques is truly tied up.
"There also is no intrinsic value to a work of art," he added.
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The paint, canvas and frame used, for instance, don't add to a painting's value. And you can't break an artwork or antique into its components for analysis, the way you can look at a public company's factories, fleets, cash flow and patents.
Collecting art or antiques also means shouldering costs for insurance, expert authentication, shipping and storage in proper conditions of heat, humidity and sunlight—expenses you don't incur with stocks and bonds.
And buying and selling artwork or antiques can entail commissions and markups that can range from 10 percent to 25 percent of the work's sales price, Yardumian said. All these expenses chew into returns.
Illiquidity makes art and antiques more akin to real estate than securities. It can easily take six months or more to get a fine painting or sculpture on the auction block, according to Moses.
And the pace of price gains is unpredictable.
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"You have to have the stomach to just leave the money there," Yardumian said, pointing out that investors should not invest in art or antiques with funds needed for college expenses, retirement or any other purpose with a deadline.
"You would not put money into art that you need to maintain your lifestyle," he advised.