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Rag matboard is of archival quality and will provide a lifetime of protection. Matboard core is white. You must use conservation (acid-free) backings and tape with these mats for effective protection.
Choosing an Opening Size
Openings are cut to the size you specify; normally you will want an opening 1/2" smaller per dimension than your artwork. This gives space to hinge and prevents it falling through the mat opening. So, for example, an 8x10 photo would use a 7 1/2 x 9 1/2 opening.