8/23/2023 0 Comments Medieval times scottsdale drewIt is this particular lack of order rather than its common transparency and brittleness which is the defining feature of glass. At an atomic level, large holes are then created in the network and the sodium ions can move into them. This results in a situation where some oxygen ions are bonded to only one silicon (which is said to be non- bridging). However, every silicon still finds it possible to be tetrahedrally bonded to four oxygens. When monovalent ions are added to the vitreous silica, such as a small amount (13-17 per cent) of sodium oxide, the balance between the oxygen and the silicon ions is upset so that not every oxygen can bond with two silicon atoms. Vitreous silica then has a continuous network of interconnected silicon and oxygen atoms but lacks the parallel planes of similar atoms which characterise a crystal. In its crystalline state the angle is 180 degrees, but in its vitreous state the bond angle can vary between 180 degrees down at least to 150 degrees and sometimes lower. Vitreous silica has the same basic structure as nearly all of the crystalline phases of silica the primary difference lies in the bond angle between the silicon and oxygen atoms which form the silica molecule. However, as glass cools from its molten state to a solid the 'normal' liquid to solid crystallisation process is bypassed and the glass solidifies without crystallising. To change from the disordered molecular arrangement of a liquid state to the regular configuration of a crystal, the individual atoms of a material must be able to migrate. The use of silver stain from about the early 14th Century gave the craftsmen even greater decorative scope. In this way highly decorative window glass could be created. Coloured glass was made either by adding metallic oxides to the original material or by painting detail onto its surface using a mixture of ground glass and iron or copper oxides, which was then fused to the body of the glass by reheating it. Soda, lime and silica are the essential constituents for making most window glass from the Middle Ages on. Other oxides present also affected the properties and the resilience of the glass to varying degrees. Where a lime-rich sand was used, calcium oxide was released as the material was heated, acting as a stabiliser, improving the durability of the material. Both soda (essentially sodium carbonate) and potash (essentially potassium carbonate) were widely used as the flux for making glass in the early Middle Ages, but glass made with potash was discovered to be more prone to deterioration and was later abandoned. The composition of the material varied according to the constituents of the sand and the type of flux used to reduce the temperature required to manipulate the glass. From medieval times until the mid 19th Century, cylinder glass and crown glass were the forms most widely used for glazing windows. Under centrifugal force and its own weight, the disk enlarged gradually to produce a fine, circular sheet of window glass known as a 'crown', which could then be divided into useable shapes. It was then reheated and carefully manipulated so that the glass opened out into a disk which was then spun rapidly in front of the fire. In essence, this process involved blowing a 'gather' or blob of molten glass to form a spherical vessel which was transferred onto a pontil (a solid iron rod), leaving one end open. Later, 'crown glass' began to be manufactured. Small fragments of window glass have been excavated from, certain Roman settlement sites in England which suggest that the Romans produced an unrefined window glass, either by casting in a mould and polishing laboriously, or by blowing cylinders which were cut open and flattened to form a rectangular sheet of glass now known as cylinder glass. In Britain production was probably first introduced by the Romans when they came here around 45BC. The earliest glass was in essence merely sand fused with the aid of a flux such as plant ashes. The manufacture of painted and stained glass has seen a transformation from the early methods of production to the refined process it is today. To understand the particular problems encountered, one must first become familiar with the material. With increasing recognition for the importance of Victorian stained glass as well as medieval examples, conservators must now, more than ever, be aware of the principles involved in the decay of this precious material and in the principles involved in its repair. Examples have survived since the Middle Ages, yet the material is also surprisingly vulnerable to decay. It is extremely hard, durable and dense, yet it is transparent, and its molecular structure displays the random disorder of a frozen liquid, not the ordered structure of a crystal. Its use to produce dazzling displays of colourful light has amazed church-goers for centuries. Glass is one of the most intriguing and complex materials found in historic buildings.
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