Photo by Christine Brandel
Shown is a prunted beaker with a foot and stem. Notice the greenish color of the glass. This was the result of iron oxide occurring in the glass batch. It distinguishes the glass made in the central and northern regions of Europe from the glass made in the Mediterranean, and southern Germany, Switzerland and Italy. The heavy forestation of the regions where waldglas was manufactured contributed to the color of the glass -- trees were used for fuel to run the furnace, and the sand from this area used in the glass batch contained the iron oxide that colored the glass green. This glass was also considered a potassium-lime glass, meaning that the chemical components that made up this glass included silica (sand), potassium carbonate (derived from wood ashes), and lime. This is different than soda-lime glass, which was made in other regions and included silica, lime and sodium oxide. Potassium-lime glass is harder, more resistant to breakage, and can be engraved and wheel-cut. It's also more brilliant, but it changes state from molten to solid more rapidly, and thus must be worked more quickly.
Here's another example of forest glass:
Photo by The Allaire Collection
Personally, I love the color of this type of glass. In later years, it was discovered that adding manganese oxide to the glass batch would result in a colorless glass, but I have always had a love for the deep foresty green of the glass of this period.