GLASS TIMEFRAME

During the Medieval Period (from the 8ᵗʰ till 16ᵗʰ century CE) in Northern and Western Europe, the raw materials for glass were Sand, Lime and Plant-ash. It is referred to as FOREST GLASS with High Potash (K) and Low Sodium (Na). At the beginning of the 16ᵗʰ century, the raw materials for glass slowly changed to Sand, Lime and Soda (Seaweed-ash) and is known as
SODA LIME GLASS (Low K, High Na).

FOREST GLASS had the disadvantage of being more brittle and less forgiving. SODA LIME GLASS has a lower melting point, is better workable and was quickly preferred by artisans. With better properties, SODA LIME GLASS completely replaced FOREST GLASS by the end of the 16ᵗʰ century.

"K appears in 5 of the glasses. This is known as Forest Glass, but 4 of the glasses show no K"

-- Dr. Bruce Kaiser