After about 1000AD there was a significant change in the composition of northern European glassware. There, the glass-makers abandoned marine plant ash from the Mediterranean countries, which they had used as a source of alkali (in the form of soda), and came to rely instead on local supplies of potash, derived from the ashes of bracken, beechwood and other woodland plants. The products made from the new potash were simple utilitarian objects such as urinals, apothecary flasks and beakers with moulded and applied decoration. They appeared in a wide range of natural green and amber tints, caused mainly by iron impurities in the raw materials, which were known collectively as Waldglas  or Forest glass . Waldglas was made in the forests of  Bohemia ,Germany, and France (where it was known as verre de fougere), until as late as the 17th century

Properties of Glass

The basic ingredients of glass were unchanged from its discovery thousands of years ago until the 17th century, when lead glass was introduced. These are silica (from sand), lime and an alkali, such as soda or potash. The alkali is added to facilitate melting and lime is used to stabilise the glass, making it less vulnerable to the damaging effects of water. To this basic mixture (known as batch) metallic oxides  can be added to colour the glass, or to make it opaque. This mixture becomes soft and malleable when heated sufficiently, allowing it to be formed, using various techniques, into a myriad of shapes and sizes. On cooling this mass becomes glass; it retains, however, the random molecular structure of a liquid.
Solids such as metal, during cooling from a molten state, set up a network of interlocking crystals, which creates a very strong substance. Glass does not do this, which is why it shatters easily when struck or dropped and why it deteriorates over time (a process known as devitrification). It is also why molten glass must be cooled in a gradual and controlled manner, in an annealing oven, in order to release internal stresses. Cooling the glass too quickly can result in shattering or cracking. Glass has another unusual characteristic in that, unlike metals which flow or set at specific temperatures, its viscosity changes as it is heated. It softens progressively as the temperature rises, becoming increasingly malleable, until it flows like thick syrup. This makes glass amenable to more heat-formed techniques than most other materials, which gives it its great versatility.