Gray cards are made of a variety of materials including fabric, plastic, paper, and foam. Some photographers hold that any neutral white or gray surface, such as a white piece of paper, a concrete or stone wall, or a white shirt are suitable substitutes for a gray card; however, since bright white papers and clothing washed in typical detergents contain fluorescent whitening agents, they are not spectrally neutral. Gray cards specially made to be spectrally flat are therefore more suitable to the purpose than surfaces that happen to be available.