Bricks as time capsules of DNA

I listen to all of the BBC Science in Action podcasts.  A recent one featured a report of how unfired clay bricks can be used to access ancient DNA.

"We extracted ancient DNA from a recently exposed fracture surface of a clay brick deriving from the palace of king Ashurnasirpal II (883–859 BCE) in Nimrud, Iraq. We detected 34 unique taxonomic groups of plants. With this research we have made the pioneering discovery that ancient DNA, effectively protected from contamination inside a mass of clay, can successfully be extracted from a 2900-year-old clay brick... in this study, it was possible to analyse the aDNA with a direct link to context and date, which allowed us to correlate with roughly contemporary textual evidence to present living plants."