(CNN) The clock is ticking on a $10 million reward for some missing artwork -- including Rembrandts, a Vermeer and sketches by Degas.

The 13 pieces of art were stolen from the Isabella Stewart Gardner Museum in Boston in a daring 1990 heist. They are valued at more than $500 million.

Initially, the museum offered a $5 million reward for the artwork. In May, the museum doubled it -- to $10 million -- in an attempt to entice anyone with information to come forward.

But the $10 million reward is only valid until December 31. When asked what the reward will be after that, Kathy Sharpless, director of marketing and communication for the museum, said, "At this point, we are just being focused on January 1."

"This was a reward that was created with a deadline in order to create a sense of urgency," Sharpless added.

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