HS2440 Week 10 Asyn_Glass structure and decay

Question 1

1. three components of glass
2. what is ‘the batch’
3. what are common sources for flux
   what material works as a stabiliser

Answer 1

sand - silica

Question 2

describe the chemical structure of glass
what is the ‘non- bridging oxygen’

what is the role of ca2++ as a stabliser

Answer 2

an oxygen molecule which doesn’t bridge to another sillicon; therefore requires one positive charge to be balanced, it uses sodium to provide the positive charge.

balances out the negative charges of two non-bridged oxygen molecules- it is more tightly attracted and harder to break that the Na- O

Question 3

two historical common glass compositions, their features and prevalence throughout time

Answer 3

1. uses burnt plant ash as flux, as sodium rich, or trees which are potassium rich when burnt
2. another source of flux is dried up river bed which are mineral-rich, soluble salts on hillsides rinsed onto river beds, can be dug out to obtain natron; sodium rich

Question 4

1. types of glass common to stained glass windows in europe; 13th to 20th century
2. flow chart showing how to classify glass based on major composition (5 different classifications of glass)

Answer 4

Question 5

what does this graph tell us about medieval window glass composition

Answer 5

north west France and England shows high levels of potassium, relative to slow sodium %

Rhine land has low % of both potassium and sodium

central europe (bohemia) has compositions of high potash compared to sodium

Question 6

what are the properties of lead glass, how can they be manipulated

Answer 6

Lead glass, commonly called crystal, is a variety of glass in which lead replaces the calcium content of a typical potash glass.

lead crystal glassware was formerly used to store and serve drinks, but due to the health risks of lead, this has become rare. One alternative material is modern crystal glass, in which barium oxide, zinc oxide, or potassium oxide are employed instead of lead oxide.

Question 7

what causes the decay of historical glass

Answer 7

its solvation of the alkali ions which causes its decay

or silica breaking down

hence, calcium is important for its long term stability

Question 8

glass decay often involves the leaching of…

describe the process (schematic)

Answer 8

Exposure of a glass surface to moisture, either in solution or from humidity in the atmosphere, causes chemical reactions to occur on and below the surface of the glass. The exchange of alkali metal ions (from within the glass) and hydrogen ions (from outside) can cause chemical and structural changes to the glass. When alkali metal cations in the near-surface layer are replaced by smaller hydrogen ions, structural differences between the affected surface layer and the unaffected lower layers of glass cause increasing tensile stress, which in turn can cause cracking.[7][8]

The likelihood of degradation due to glass disease depends on the amount and proportion of alkaline compounds mixed with silica, and on surrounding conditions.[3] Inadequate calcium oxide causes the alkalis in the glass to remain water-soluble at a low level of humidity. Exposure to higher levels of relative humidity during storage or display causes alkali to hydrate and leach out of the glass.

Question 9

when glass is leached, what are the effects on the properties of the material?

Answer 9

change in refractive index; more air in the glass, change in density, because of size of ions

form a crust of the surface- frostiness; caused by calcium carbonates and alkali on the surface eg. potassium carbonate

(moisture causes alkali to be leached out of the glass. This becomes apparent when hygroscopic alkali deposits on the glass give it a cloudy or hazy appearance.[7][13] This may occur within as little as five to 10 years of the glass’s manufacturing. The glass may feel slippery or slimy[10] and tiny droplets, or weeping, may be seen in high humidity (above 55%).[14] The hydrated alkali can form fine crystals on the surface of the glass in low relative humidity (below 40%)).

Question 10

evidence and effects of advancing decay in glass

Answer 10

Question 11

why might some glassware be particulary unstable, such as 16th and 17th century venetian glassware

Answer 11

very high potassium oxide levels
form beads of moisture, as its hydroscopic at high RH

due to purification processes, high potassium/ flux, and decreased calcium stabiliser

Question 12

archeological glass features, as seen in photo

Answer 12

flaky crust post excavation

retains fine upper leached area as loses flake due to drying environment

pitted surface; uneven leaching in archeological environment

optical (iridescent effect) from leaching

(image) SEM images of leached archeological glass, mapping concentrations of elements

Question 13

(image) crusts and staining on archaological glass. can cause complete lamination. what might cause there to be no pristine glass left

Answer 13

manganese staining. comes from the burial environment or within the content of the glass.