Optical brightener

Optical brighteners, also called fluorescent whitening agents or FWAs, are compounds applied during fabric finishing that absorb invisible UV light and re-emit it as visible blue light, countering raw cotton's natural yellowish cast so white fabric reads bright white — sometimes with a faint glow under blacklight or camera flash. They're standard finishing on nearly every off-the-shelf white blank sold at retail.

That same chemistry works against a shop doing tie-dye, ice-dye, or sublimation: brighteners sit on the fiber surface and resist or unevenly accept dye, and on sublimation blanks they can visibly dull or gray out a transferred image where the design called for pure white. We printed a sublimation run on off-brand poly blanks once without checking the spec sheet, and the whites came back with a faint bluish cast under shop lighting that customers noticed before we did.

Buy PFD or brightener-free blanks specifically labeled for sublimation or hand-dyeing whenever whites in the final art matter, and don't assume a spec sheet listing white means brightener-free — ask the supplier directly if it isn't stated outright.

The instinct is to assume a brightener-free blank will look dingy next to a standard one on the shelf. It does read slightly warmer in daylight — a fair trade for a dye or sublimation job coming out clean instead of blotchy or discolored.