I have to restore an outdoor sign that is made of a black agglomerate marble, the sign is faded and is very gray. I performed a small test patch and I was successful at restoring the sign. The question I have is, what caused the agglomerate to fade a

Agglomerate marbles are made of a combination of marble chips and a polyester resin. Polyester resins are very sensitive to ultraviolet light(UV) and therefore should not be used outdoors. To answer your question, yes, the marble will fade in a very short time. An application of a UV resistance coating will prolong the fading but will eventually fade again. Agglomerate marbles should never be used on exteriors.

TIP Agglomerate marbles will typically have an uneven shine across its surface. The polyester matrix will fade first turning a milky color.