Glasswork - the Scientia Review

The Art of
Glasswork
By
CHRIS STARBARD
and
BECCA CHURCH

Flashed, Cased,
and Stained
Three kinds of colored glass called flashed, cased, and
stained glass, are named after the methods used to color them.
Flashed and cased glass are similar in that the color is applied
through a separate of glass, while stain is painted on. These
processes will be explained in more detail on the next page.
Flashed
Glass
Cased
Glass
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Stained
Glass

All processes begin with an annealed, or
cooled, colorless glass object. Flashed glass is colored when
a clear glass object is dipped into molten colored glass to be
coated in a thin layer. Cased glass also has layers of colored
glass added on, but usually multiple layers of different colors
are applied. Then areas of the glass are cut into to reveal
different colors. Stained glass is created when the glass
object is coated with a chemical stain. Then the glass is fired
and the color of the chemical is developed by the heat.
An elaborate
example of cased
glass in which
the blue glass
was chiseled and
cut away in a
pattern to reveal
the white.
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Coloring with Metallic
Salts
Minerals and purified metallic salts are commonly
used as pigments to color glass. Natural impurities in the
sands or smoke involved in glassmaking can cause discoloration
in glass, as was often the case with earlier glass. For example,
iron in the sand and sulfur in the smoke of the burning coals
burned to make English “black bottle glass” in the 17 th century
produced a dark hue in the glass. However, nowadays these
impurities are often removed to make clear glass to be colored
only by selected metallic salts. Manganese dioxide and cerium
oxide are commonly used decolorizers, chemicals that precipitate
out, or cause to be deposited in solid form from a liquid, the
impurities.
Black Bottle
Glass from the
17 th Century
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Chemistry of Color
Each chemical used to color glass develops its own distinct
color and some also remove other colors due to impurities. Iron oxides
tend to create green and brown colors. Manganese oxides are used to
remove greens but can also develop a deep amber or amethyst color in
larger amounts. Reds can by formed by gold chloride, selenium
compounds, and some copper compounds. Whites can be derived from
glass with tin compounds or antimony oxides, while black comes from a
mix of manganese, cobalt, and iron. Cobalt oxide and copper
compounds can lead to blue glass. Carbon oxides and sulfur
compounds cause glass to be amber or brown. Perhaps the most
fascinating chemically formed color is the yellow-green glass produced
by the addition of uranium oxides – this glass actually glows under
ultra-violet light!
Uranium
glass
glowing
under a
UV light
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Fusing the Color
Stained glass, as
described on page 12, must be
fired to develop the color of the
stain and fuse it with the glass.
The heating process required to
seal the color to the glass is
carried out in a kiln, or an oven or
The Lehr
A lehr is a type of kiln
used specifically for
glasswork. It is used
for the process or
annealing glass. It
helps stabilize the
temperature distribution
in glass objects to
reduce the stress on the
glass and therefor
prevent cracking.
thermally insulated chamber used
to control temperatures necessary
for various processes, such as
annealing glass.
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Preservation
The breakdown of glass depends most directly upon
composition. Glass contains silica, alkali, and minute amounts of
calcium as primary constituents, and these ingredients each effect the
breakdown process Though glass is somewhat uniform in
composition, small differences in makeup account for large differences
in the breakdown process. Some types of Roman glass, for example,
are extremely resistant to breakdown, unlike much medieval glass
which degenerates with ease due to a high potash content; the potash
from beech wood renders the composition unstable.
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Water is the most primary mechanism of decomposition.
It forms a number of acidic solutions with ease and without special
conditions. The resultant mild acids withdraw alkali from the silica
structure of the glass. The removal of this substance renders the core
material weak, but also harms the surface; at the surface, alkaline
compounds actually retain water and catalyze the degradation
process. When glass reaches this state, the surface often feels
slippery, and this effect is referred to as weeping. The alkali removal
causes visible cracks to develop which account for a cloudy
appearance. Heightened alkali withdrawal results in flaking.
Extreme thermal fluctuation also poses a threat by creating forces
between the glaze and base of a glass, often separating these adjacent
layers.
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History of Leaded Glass
The use of lead additives in glassmaking originates in
Mesopotamia, which scholars generally agree upon as the origin
of the glass craft. Recipes for lead-based glass have been found
inscribed upon Mesopotamian in addition to actual colored
fragments. Interestingly, recovered shards display a relatively
low amount of lead, and it is possible that in some case the
inclusion of lead was entirely accidental or experimental. Ancient
lead glass discovered in China is much newer and suggests that
practice is not as deeply rooted. Medieval glassworkers added
Lead compounds intentionally in the greatest amounts; these
substances proved well suited not only for painting stained glass
but also for integral pigmentation.
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Lowered viscosity and working temperature rendered
lead as an advantageous addition to glass; period technology could
not provide high temperatures or function well with excessively
hot material. George Ravenscroft revolutionized the glassmaking
trade by using flint in large scale glass production. Eventually,
his product utilized sand rather than flint and contained a
significant concentration of lead oxide. The result was the clear
lead crystal which now typifies leaded glass. He received several
patents for his work, and provided an alternative to the shoddy
material supplied by Italy.
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Chemistry of Leaded Glass
Lead oxide, as an additive in glass production, is highly
effective because it causes the material to be more fluid when molten
and lowers the temperature at which the material may be
manipulated. Those qualities were highly advantageous
historically because antique technology could not provide
excessively high temperatures and because more viscous material
required less effort to form. Lead is not added strictly as lead but
rather as lead oxide. As an oxide, it prevents the deterioration of
alkali structures within the glass and thus prevents the major
cause of breakdown.
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Glossary
Anneal – To heat and then cool glass to increase the
strength and durability of the material. (p. 12)
Cased Glass – Clear glass with layers of colored glass
applied and chipped away. (p. 12)
Decolorizers – Chemicals that precipitate out impurities
that cause discoloration. (P. 13)
Flashed Glass – Clear glass dipped in a layer of colored
glass. (p. 12)
Kiln - An oven or thermally insulated chamber used to
control temperatures necessary for various processes. (p.
15)
Lead Cames – Grooved lead bars that hold together glass and
sometimes act as panes in stained glass windows. (P. 3)
Leadlights – Sections of glass supported by lead cames,
often as decorative windows. (p. 4)
Medium – The category of a work of art or the materials used
to create it. (P. 2)
Precipitate – cause to be deposited in solid form from a
liquid (P. 13)
Solder – Two use a low-melting combination of metals to join
metals that do not melt as easily. (P. 3)
Stained Glass – Glass onto which a stain is painted and then
the entire glass item is heated to fuse the stain to the
glass. (P. 12)
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About the
Authors
Chris Starbard is a hard-working,
friendly mass academy student from
Oakham, Massachusetts. In his free
time he enjoys working on,
collecting, and riding antique
bicycles.
Becca Church is a bubbly Mass Academy
student from Northborough,
Massachusetts. Her hobbies include
singing, drawing, writing poetry, and
playing the piano.
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Illustration Credits
Page 1 – Wikipedia.org
Page 2 – flickriver.com
Page 3 – Wikipedia.org
Page 19 – vam.ac.uk
Page 20 –apartmenttherapy.com
Page 4 – classicglass.com
Page 5 – scottspeck.com
Page 7 – en.murano-glass-shop.it
Page 8 – theageofelegance.com
Page 9 – glazette.com
Page 10 – mdiglass.com
Page 11 – patternglass.com
Page 12 – antiquie-marks.com
Page 13 – antique-bottles.com
Page 14 – Wikipedia.org
Page 15 – vintage-technology.info
Page 16 – yorkminster.org
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