This is not an English word. From what I can tell, it's an Italian technical term used in sculpture.

From a Google search, I think "allustratura" means "shiny" (sparkling, bright, luminous, etc.) "The diamond ring sparkled and shined in the light." "The new penny was bright and shiny." https://books.google.com/books?id=KYSUBgAAQBAJ&pg=PA134&lpg=PA134&dq=allustratura+in+english&source=bl&ots=9bWEBq0SE7&sig=DRt2kqOOBbf7qVmbImCogSp5xV4&hl=en&sa=X&ei=qVqVVaOIOMb5-AHvwI34BA&ved=0CCEQ6AEwAA#v=onepage&q=allustratura%20in%20english&f=false