Glazing and their types

GLAZING 

TYPES OF GLASS:

(1) Soda-lime glass

(2) Potash-lime glass

(3) Potash-lead glass

(4) Common glass

(1) Soda-lime glass:

This is also known as soda-glass or soft glass. It is mainly a mixture of sodium silicate and

calcium silicate.

Properties:

(i) It is available in clean and clear state.

(ii) It is cheap.

(iii) It is easily fusible at comparatively low temperature.

Uses: It is used in the manufacture of glass tubes and laboratory apparatus, plate glass,

window glass, etc.

(2) Potash-lime glass:

Also known as bohemian-glass or hard glass.It is mainly a mixture of potassium silicate

and calcium silicate.

Properties:

(i) it fuses at high temperature.

(ii) it is not easily affected by water and other solvents.

(iii) it does not melt so easily.

Uses: used in manufacture of glass articles.

(3) Potash-lead glass:

Also known as flint glass.It is a mixture of potassium silicate and lead sillacate.

Properties:

(i) Fuses very easily.

(ii) Easily attacked by aqueous solution.

(iii) Posses great refractive power.

(iv) Specific gravity is about 3 to 3.50.

(v) Turns black and opaque.

Uses: used in the manufacture of artificial gems, electric bulbs, lences, prisms etc.

(4) Common glass

Also known as bottle glass. Manufacture of sodium silicate, calcium silicate and iron silicate.

Properties:

(i) Fuses with difficulty.

(ii) It is brown, grey or yellow in colour.

(iii) easily attacked by acids.

Uses: it is mainly used for medicine bottles.

MANUFACTURING OF GLASS:

1. Batch processing system (batch house):

Batch processing is one of the initial steps of the glass-making process. The batch house

simply houses the raw materials in large silos (fed by truck or railcar) and holds anywhere

from 1–5 days of material. Some batch systems include material processing such as raw

material screening/sieve, drying, or pre-heating (i.e. cullet). Whether automated or manual,

the batch house measures, assembles, mixes, and delivers the glass raw material recipe

(batch) via an array of chutes, conveyors, and scales to the furnace. The batch enters the

furnace at the 'dog house' or 'batch charger'. Different glass types, colors, desired quality, raw

material purity / availability, and furnace design will affect the batch recipe.

The hot end of a glassworks is where the molten glass is formed into glass products,

beginning when the batch is fed into the furnace at a slow, controlled rate by the batch

processing system (batch house). The furnaces are natural gas- or fuel oil-fired, and operate at

temperatures up to 1,575°C. The temperature is limited only by the quality of the furnace’s

superstructure material and by the glass composition. Types of furnaces used in container

glass making include 'end-port' (end-fired), 'side-port', and 'oxy-fuel'. Typically, furnace

"size" is classified by metric tons per day (MTPD) production capability.

Forming process

There are, currently, two primary methods of making a glass container: the blow and

blow method, used for narrow-neck containers only, and the press and blow method used for

jars and tapered narrow-neck containers.

In both methods, a stream of molten glass, at its plastic temperature (1050°C-1200°C), is cut

with a shearing blade to form a solid cylinder of glass, called a gob. Both processes start with

the gob falling, by gravity, and guided, through troughs and chutes, into the blank moulds,

two halves of which are clamped shut and then sealed by the "baffle" from above.

In the blow and blow process the glass is first blown through a valve in the baffle, forcing it

down into the three piece "ring mould" which is held in the "neckring arm" below the blanks,

to form the "finish", [The term "finish" describes the details (such as cap sealing surface,

screw threads, retaining rib for a tamper-proof cap, etc.) at the open end of the container.]

Containers are made in two major stages. The first stage moulds all the details ("finish")

around the opening, but the body of the container is initially made much smaller than its final

size. These partly manufactured containers are called parisons, and quite quickly, they are

blow-molded into final shape.

Referring to the mechanism, the "rings" are sealed from below by a short plunger. After the

"settleblow" finishes, the plunger retracts slightly, to allow the skin that's formed to soften.

"Counterblow" air then comes up through the plunger, to create the parison. The baffle rises

and the blanks open. The parisonis inverted in an arc to the "mould side" by the "neckring

arm", which holds the parison by the "finish".

As the neckring arm reaches the end of its arc, two mould halves close around the parison.

The neckring arm opens slightly to release its grip on the "finish", then reverts to the blank

side. Final blow, applied through the "blowhead", blows the glass out, expanding into the

mould, to make the final container shape.

In the press and blow process, the parison is formed by a long metal plunger which rises up

and presses the glass out, in order to fill the ring and blank moulds.[5] The process then

continues as before, with the parisonbeing transferred to the final-shape mould, and the glass

being blown out into the mould.

The container is then picked up from the mould by the "take-out" mechanism, and held over

the "deadplate", where air cooling helps cool down the still-soft glass. Finally, the bottles are

swept onto a conveyor by the "push out paddles" that have air pockets to keep the bottles

standing after landing on the "deadplate"; they're now ready for annealing.

The forming machines hold and move the parts that form the container. The machine consist

of basic 19 mechanisms in operation to form a bottle and generally powered by compressed

air (high pressure - 3.2 bar and low pressure - 2.8 bar), the mechanisms are electronically

timed to coordinate all movements of the mechanisms. The most widely used forming

machine arrangement is the individual sectionmachine (or IS machine). This machine has a

bank of 5–20 identical sections, each of which contains one complete set of mechanisms to

make containers. The sections are in a row, and the gobs feed into each section via a moving

chute, called the gob distributor. Sections make either one, two, three or four containers

simultaneously. (Referred to as single, double, triple and quad gob). In the case of multiple

gobs, the shears cut the gobs simultaneously, and they fall into the blank moulds in parallel.

COMPOSITION OF GLASS

The following is a list of the more common types of silicate glasses, and their ingredients,

properties, and applications:

1. Fused quartz, also called fused silica glass, vitreous silica glass, is silica (SiO2) in

vitreous or glass form (i.e., its molecules are disordered and random, without

crystalline structure). It has very low thermal expansion, is very hard, and resists high

temperatures (1000–1500 °C). It is also the most resistant against weathering (caused

in other glasses by alkali ions leaching out of the glass, while staining it). Fused

quartz is used for high temperature applications such as furnace tubes, lighting tubes,

melting crucibles, etc.

2. Soda-lime-silica glass, window glass: silica 72% + sodium oxide (Na2O) 14.2% +

lime (CaO) 10.0% + magnesia (MgO) 2.5% + alumina (Al2O3) 0.6%. Is transparent,

easily formed and most suitable for window glass (see flat glass). It has a high

thermal expansion and poor resistance to heat (500–600 °C). It is used for windows,

some low temperature incandescent light bulbs, and tableware. Container glass is a

soda-lime glass that is a slight variation on flat glass, which uses more alumina and

calcium, and less sodium and magnesium which are more water-soluble. This makes

it less susceptible to water erosion.

3. Sodium borosilicate glass, Pyrex: silica 81% + boric oxide (B2O3) 12% + soda

(Na2O) 4.5% + alumina (Al2O3) 2.0%. Stands heat expansion much better than

window glass. Used for chemical glassware, cooking glass, car head lamps,

etc. Borosilicate glasses (e.g. Pyrex) have as main constituents silica and boron oxide.

They have fairly low coefficients of thermal expansion (7740 Pyrex CTE is 3.25×10–

6/°C[4] as compared to about 9×10−6/°C for a typical soda-lime glass[5]), making them

more dimensionally stable. The lower CTE also makes them less subject

to stress caused by thermal expansion, thus less vulnerable to cracking from thermal

shock. They are commonly used for reagent bottles, optical components and

household cookware.

4. Lead-oxide glass, crystal glass: silica 59% + lead oxide (PbO) 25% + potassium

oxide (K2O) 12% + soda (Na2O) 2.0% + zinc oxide (ZnO) 1.5% + alumina 0.4%.

Because of its high density (resulting in a high electron density) it has a high

refractive index, making the look of glassware more brilliant (called "crystal", though

of course it is a glass and not a crystal). It also has a high elasticity, making glassware

'ring'. It is also more workable in the factory, but cannot stand heating very well.

5. Aluminosilicate glass: silica 57% + alumina 16% + lime 10% + magnesia 7.0% +

barium oxide (BaO) 6.0% + boric oxide (B2O3) 4.0%. Extensively used for fiberglass,

used for making glass-reinforced plastics (boats, fishing rods, etc.) and for halogen

bulb glass.

6. Oxide glass: alumina 90% + germanium oxide (GeO2) 10%. Extremely clear glass,

used for fiber-optic waveguides in communication networks. Light loses only 5% of

its intensity through 1 km of glass fiber.[6] However, most optical fiber is based on

silica, as are all the glasses above.

PROPERTIES OF GLASS

The properties of glass are mainly governed by factors like composition of the constituents,

state of surface, thermal treatment conditions, dimension of specimen etc.

Following are the properties of glass which have made the glass popular and useful:

I. It absorbs, refracts or transmits light.

II. It can take up a high polish and may be used as substitute for every costly gems.

III. It has no definite crystalline structure.

IV. It has no sharp melting point.

V. It is affected by alkalis.

VI. It is an excellent electrical insulator at elevated temperatures due to the fact that glass

can be considered as an ionic liquid. The ions are not easily moved at room

temperature because of the high viscosity. But when the temperature rises, the ions

are permitted to flow and thus they will sustain an electric current.

VII. It is available in beautiful colours.

VIII. It behaves more as a solid than most solids in the sense that it is elastic. but when the

elastic limit is exceeded, it fractures instead of deforming.

IX. It is capable of being worked in many ways. it can be blown,drawn,or pressed. But it

is strange to note that it is difficult to cast in large pieces.

X. It is extremely brittle.

XI. It is not usually affected by air and water.

XII. It is not attacked by ordinary chemical reagents.

XIII. It is possible to intentionally alter some of its properties such as

fusability,hardness,refractive power etc. To suit different purposes.

XIV. It is possible to obtain glasses with diversified properties. The glass may be clear,

colourless, diffused and stained.

XV. It is possible to weld pieces of glass by fusion.

XVI. It is transparent and translucent. Transparency is the most used characteristic of glass

and it is due to the absence of the free electron. For the same reason it also works as a

good insulator.

XVII. When it is heated , it becomes soft and soft with rise in temperature . it is ultimately

transformed into a mobile liquid. The liquid when allowed to cool , passes to all

deegres of viscosity. The property of glass has made its manufacturing process easy.

It can also be formed into articles of desired shape. Thus the amorphousness of glass

permits it to be blown, drawn from furnaces and continuously worked.

XVIII. Due to advancement made in the science of the glass production , it is possible to

make glass lighter than cork or softer than cotton or stronger than steel. The presence

of glass however is considerably affected by foreign inclusions , internal defects and

cords or chemically heterogeneous areas.

XIX. The glass panes can be cleaned easily by anyone of the following methods-

[i]By applying methylated spirit

[ii]painting the glass panes with lime wash and leaving it to dry and then washing

with clean water.

[iii]rubbing damp salt for cleaning paint spots and;

[iv]rubbing finely powdered

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