Photo sensitivity means light will change this product over time – how quick depends on how much light it gets. In this case the product will get slightly lighter (lighter greys, creamy tones)in shade and that is what is happening. However, the black lines were always there, as it is “Fill” used to fill crack, splits, knot holes etc. in the plank surface during the manufacturing process. This is part of what give the floor the distressed character is has. Although fill can come in many colors most manufacturers use black fill to accentuate the distressing. So the black is standing out more because the fill doesn’t change too much in color (maybe it fades slightly) while American walnut hardwood will lighten the fill doesn’t lighten as much. It was always there but just may be a little more noticeable. It is part of the characteristic of the floor.
While all hardwoods are photosensitive to some degree, certain types are more sensitive than the rest.
For example, when first cut, walnut can range from a light pale brown to a dark chocolate brown. When exposed to sunlight, however, a dark brown walnut can lighten to a golden brown tone overtime.
Another wood that’s more prone to photosensitivity is tulipwood, also known as Poplar. When first cut, the color ranges from a green to a purple tone and can transform to a warm milk chocolate brown overtime.
Cherry wood, both American and exotic species, is the most photosensitive of hardwoods. It will change from a pink tone to a red-brown tone overtime.