I am NOT a chemist.

So I can’t even begin to get to the HOW of this.

Maybe someone else can?

How did he make the veil? How is it that you see the features of the Saviour beneath the veil? Did Sanmartino sculpt it that way? How is that possible? One hypothesis is that the finished statue was covered with a cloth and that the cloth was permeated with a solution that crystallized as calcium carbonate, creating the veil. Only Sanmartino knows for sure.