Today’s objectives-glass and clay products and processing

What are standard glass additives, and how do they effect structure and properties?

How do specific volume and viscosity vary with temperature?

How are glass sheets and containers prepared?

Why are annealing and tempering important for glass?

What are the steps in processing clays?

What is slip, and what is hydroplasticity?

Describe casting

Describe extruding

Describe drying

Describe firingPrimary glass resource: Corning

Museum of Glass (www.cmog.org)

History of GlassThe history of the origin of glass can be categorized by

periods according to the methods of the manufacturing process as follows:

• The First Period: 1700 BC through 100 AD– Primitive method of making glass using molds.

• The Second Period: 100 AD through 400-500 AD– Glassblowing technique discovered, and glass manufacture

becomes a more practical process.

• The Third Period: 4-500 AD ~ 1200 AD– Middle Ages, characterized by Byzantine glass.

• The Fourth Period: 1200 AD ~ 1900 AD– Venetian glass, foundation for modern glass making is set.

• The Fifth Period: 1900 AD ~ present– Glass objects used as everyday goods; large scale manufacturing.

Mesopotamian glassAs early as 3,300 years ago, secret "instructions" for furnace building

and glassmaking in Mesopotamia were written on clay tablets in a cuneiform script. Typical instructions include:

• When you set up the foundation of a good furnace to make glass, first search in a favorable month for a day of good omen…

• You regularly perform libation offerings (drink of honey and liquid butter honoring a deity).

• On the day when you plan to make (glass), you make a sheep sacrifice.

• Place juniper incense on the incense burner• Pour out a libation; only then can you make the fire in the hearth

of the furnace and place the glass in the furnace. • The wood that you burn in the hearth of the furnace should be

thick, peeled poplar wood, which has no knots, bound together with leather straps, cut in the month of the Abu (Jul. or Aug.).

• If you want to produce zagindurû-colored (blue) glass, you finely grind separately:

– 10 minas (about one pound) of immanakku-stone (quartz)– 15 minas of naga-plant ashes– 1 2/3 minas of 'white plant.' – Mix these together. – Place the mixture into a cold furnace that has four openings, – As soon as the mixture glows yellow, you pour it on a kiln fired brick

and this is called zukû-glass....

Glasses and glass ceramics

• Now commonly applied for:– Containers– Windows– Decoration– Lenses– Fiberglass (insulation)– Fiberoptics– Road signs– Composites

• Chemically, glass is comprised of noncrystalline silica plus additives:– CaO

– Na2O

– K2O

– Al2O3

• Basic Unit:

Si04 tetrahedron4-

Si4+

O2-

• Quartz is crystalline SiO2:

• Glass is amorphous• Amorphous structure occurs by adding impurities

(Na+,Mg2+,Ca2+, Al3+)• Impurities: interfere with formation of crystalline structure.

Si4+

Na+

O2-

(soda glass)Adapted from Fig. 12.11, Callister, 6e.

GLASS STRUCTURE

Glass

• Glass composition determines properties• noncrystalline

glass SiO2 Na2O CaO Al2O3 B2O3 other

Fused silica >99.5%

Pyrex (borosilicate) 81 3.5 2.5 13

Container (soda-lime) 74 16 5 1 4 MgO

fiberglass 55 16 15 10 4 MgO

Thermal expansion coefficient of pyrex is 1/3 that of standard silica glass.

• Specific volume (1) vs Temperature (T):

Glass (amorphous solid)

T

Specific volume

Liquid (disordered)Supercooled

Liquid

Crystalline (i.e., ordered) solid

TmTg

• Glasses: --do not crystallize --spec. vol. varies smoothly with T --Glass transition temp, Tg

• Crystalline materials: --crystallize at melting temp, Tm

--have abrupt change in spec. vol. at Tm

Adapted from Fig. 13.5, Callister, 6e.

GLASS PROPERTIES

Amorphous materials (glasses)• For non-crystalline ceramics, deformation occurs by

viscous flow.– The deformation rate is proportional to the applied stress.

cediswithchangevelocity

stressshear

dydv

AF

dydv

tan

• Common units are N*s/m2 = Pa*s (or Poisies=0.1Pa*s)– Viscosity unit, giving the inverse of how well the material flows.

• Water at 25 °C is 0.001 Pa*s.• Motor oil (SAE10W…) is 0.132 at 25 °C.• Glass is 1014 Pa*s = 1015 Poisies (a crystal is infinite).

• Viscosity decreases with T - (sample flows easier)• Impurities lower Tdeform

GLASS VISCOSITY VS T AND IMPURITIES• Viscosity: --relates shear stress & velocity gradient: --has units of (Pa-s)

dvdy

velocity gradient

dvdy

glass dv

dyV

isco

sity

[Pa

s]

1

102

106

1010

1014

200 600 1000 1400 1800 T(°C)

Tdeform: soft enough to deform or “work”

annealing range

fused silica

96% silica

Pyrex

soda-lime

glass

Adapted from Fig. 13.6, Callister, 6e.(Fig. 13.6 is from E.B. Shand, Engineering Glass, Modern Materials, Vol. 6, Academic Press, New York, 1968, p. 262.)

Tmelt

Glass plates

• Originally, glass plates made one at a time using the Pittsburgh process (vertically draw a continuous sheet of glass of a consistent width from the tank).

• Pilkington discovered how to make glass continuously.

Float Glass, Pilkington, 1950.

• Originally able to make only 6mm thick glass, now made as thin as 0.4mm and as thick as 25mm.

• There are around 260 float plants worldwide with a combined output of about 800,000 tonnes of glass a week.

• A float plant, which operates non-stop for between 11-15 years, makes around 6000 kilometres of glass a year.

http://www.pilkington.com/pilkington/corporate/english/education/float+process/default.htm

The Floating part

• Molten glass, at approximately 1000ºC, is poured continuously from a furnace onto a shallow bath of molten tin.

• It floats on the tin, spreads out and forms a level surface.• Thickness is controlled by the speed at which solidifying

glass ribbon is drawn off from the bath. • After annealing (controlled cooling) the glass emerges as a

'fire' polished product with virtually parallel surfaces.

• Annealing: --removes internal stress caused by uneven cooling.

HEAT TREATING GLASS

further cooledbefore cooling surface cooling

tensioncompression

compressionhot hot

cooler

cooler

• Tempering: --puts surface of glass part into compression --suppresses growth of cracks from surface scratches. --sequence:

Chemical tempering is

also possible.

• Pressing:

PARTICULATE FORMING

CEMENTATIONGLASSFORMING

Gob

Parison mold

Pressing operation

• Blowing:

suspended Parison

Finishing mold

Compressed air

Adapted from Fig. 13.7, Callister, 6e. (Fig. 13.7 is adapted from C.J. Phillips, Glass: The Miracle Maker, Pittman Publishing Ltd., London.)

Fabricating Glass in other shapes

‘Crizzling’• Prolongued exposure to chemical attack

can lead to local microcracks, ‘Crizzling.’• Eventually leads to failure.

Ceramic History• In 221 BC during the Qing Dynasty, Qin Shi Huang was the

Emperor in reign. The Terra Cotta Army was built as a way of creating an illusion of strength and manpower.

• Discovered in 1974.

Most soldiers standing in formation, many with their horses ready for battle. 

en.wikipedia.org/wiki/Terracotta_Army

7000 ‘soldiers’ thus far discovered

Clay Products• Clay is an abundant raw material• When mixed with water it forms a plastic mass

– Easily shaped– Easily processed

• Structural clay products– Bricks– Tiles– pipes

• Whitewares– Only whiten after high temperature firing

• Porcelain

• Pottery

• Tableware

• China

• Plumbing “sanitary ware” = toilets

How porcelain came to Europe• 1701, August der Starke, King of Saxony and Poland, hires Johann Friedrich

Böttger into his castle workshop as an alchemist to make gold. – The alchemist cannot make gold (surprise…)

– Instead, in 1709, he is the first Westerner to discover how to make porcelain.

– The castle gets bigger (near Dresden).

• Symbolized the mysterious Far East (but made in China since 7th century). – Chinese porcelain began reaching Europe around the 13th/14th century.

– Europeans tried for years to make their own, leading to faience, majolica and bone china. But not porcelain.

– The secret lies in firing mineral quartz, feldspar, and kaolin at such a high temperature that they actually melt and fuse.

• Meissen recognized the need to protect their porcelain against imitations, so they began painting the famous "Crossed Swords" under the glaze on all Meissen porcelain as a distinguishing mark. – This is the oldest commercial trademark in

continuous use.

Ingredients

Kaolin: is a clay mineral with the chemical composition Al2Si2O5(OH)4. It is a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra.

Feldspar: is the name of a group of rock-forming minerals which make up as much as 60% of the Earth's crust. Feldspars crystallize from magma in both intrusive and extrusive rocks, and they can also occur as compact minerals, as veins, and are also present in many types of metamorphic rock. Rock formed entirely of plagioclase feldspar (see below) is known as anorthosite. Feldspars are also found in many types of sedimentary rock.

Quartz: belongs to the hexagonal rhombohedral crystal system, and is made up of silica (SiO2) tetrahedra.

Meissen Porzellan-Manufaktur • Design/model• Fabrication (hydroplastic)

– Forming– Casting– Pressing

• Shaping– Embossing

• Drying (green)• First firing• Decorating

– Under painting (monotone, more durable)– Over painting (colors, delicate work, more vivid)

• Final firing• Selling ($$$)

http://www.meissen.de

• Clay (inexpensive)• Milling and screening: achieve desired particle size. • Adding water to the clay particles produces a "slip:" --layered structure --allows material to shear easily along weak van der Waals bonds

Hydroplastic --enables extrusion --enables slip casting --enables pottery wheel (Ghost)

• Structure ofKaolinite Clay

weak van der Waals bonding

charge neutral

charge neutral

Si4+

Al3+

-OHO2-

Shear

Shear

Adapted from Fig. 12.14, Callister 6e.(Fig. 12.14 is adapted from W.E. Hauth, "Crystal Chemistry of Ceramics", American Ceramic Society Bulletin, Vol. 30 (4), 1951, p. 140.)

What are basic ceramics made of?

Slip Casting

• Pour slip into a mold (e.g. plaster of paris).

• The mold is extremely water absorbing.

• As water is drawn from the slip into the mold, the remaining part has some structural integrity (hydroplasticity at work).

• Remove the mold.• Place in the oven to drive out

the rest of the water (‘green state’).

• Fire to harden• Add glaze, fire again to vitrify it.

Extrusion

• Start with the same slip.• Extrude, taking advantage of hydroplastic characteristic.• Dry and fire the same as for molded structures.

Adapted from Fig. 11.7, Callister 6e.

ram billet

container

containerforce

die holder

die

Ao

Adextrusion

wet slip partially dry “green” ceramic

• Drying: layer size and spacing decrease.Adapted from Fig. 13.11, Callister 6e.(Fig. 13.11 is from W.D. Kingery, Introduction to Ceramics, John Wiley and Sons, Inc., 1960.)

Drying

Increasing shrinkage

•Thick ceramic wares are likely to crack, since water is easily drawn out of the surface but not the center of a ceramic part. •Thus, there is shrinkage (densification) on the outside but not on the inside. •This is not an issue for large particles, but as surface area increases (smaller particles) more water can be removed faster, and thus there are increased drying stresses.

Firing --T raised to (900-1400 C, much higher than drying temperatures) --vitrification: glass forms from clay and flows between SiO2 particles.

• Increasing vitrification leads to shrinkage as more liquid will be present at the firing temperature.

• Strength increases with vitrification since glass fills in the pores where the water was before drying, thus fewer bad flaws for stress concentration, Weibull, etc.

• Conductivity increases with vitrification as glass has a higher conductivity than (air filled) pores that it filled.

• Corrosion resistance improves.

Vitrification is a function of:

• Composition

• Temperature

• Time at temperature

Microstructure development during firing

• Quartz grains (large dark particles)

• Surrounded by glass• Mullite needles• Pores (dark holes with

white borders)• Cracks in the quartz

that weren’t there at the start (different thermal coefficients build up stress upon cooling, leading to cracks)

SUMMARY• Glass forming and properties• Clay forming and properties• History of each

Reading for next classPowder, refractory, abrasives, and advanced

ceramics and processing

Chapter sections 13.4-7, 13.10-11