Chemically strengthened glass

Chemically strengthened glass is a type of glass that has increased strength as a result of a post-production chemical process. Chemical strengthening is the name given to glass products that have been strengthened by means of an ion-exchange process. It is a surface treatment which occurs at a temperature lower than glass melting temperature. The process is particularly useful for thin glass, tiny glass and shape glass which cannot be tempered by ordinary physical tempering.

Chemically strengthened glass is typically six to eight times the strength of float glass. In the case of breakage, chemically strengthened glass breaks into bigger pieces which are not as sharp as those of non-toughened glass. The surface compression condition which is higher in the case of a chemically strengthened glass also involves an increase of flexion resistance, which is one of the main characteristics of chemically strengthened glass.

Chemical strengthening results in a strengthening similar to toughened glass. Chemically strengthened glass has little or no bow or warp, optical distortion or strain pattern. This differs from toughened glass, in which slender pieces can be significantly bowed.

Chemically strengthened glass may be cut after strengthening, but loses its added strength within the region of approximately 20 mm of the cut. Similarly, when the surface of chemically strengthened glass is deeply scratched, this area loses its additional strength. Chemically strengthened glass retains its colour and light transmission properties after treatment.

Chemically strengthened glass offers an improved scratching, impact and bending strength, as well as an increased temperature stability.

Manufacturing process

The glass is chemically strengthened by a surface finishing process. The glass to be treated is dipped into a bath of dissolved potassium salts at a temperature about 380oC for duration from 4 to 30 hours, producing an ionic exchange between the superficial sodium ions in the glass and potassium ions inside the bath. The cycle time would be greatly reduced if the glass is made of certain elements such as lithium or magnesium because ion mobility between potassium and these elements is a lot faster. The process parameters such as ion exchanging time and temperature would be modified according to the type of glass to be treated and the required strengthen specification.

The introduction of potassium ions which are larger in size than the sodium ions results in the establishment of a system of residual stress characterized by compression stretches on the surface counterbalanced by traction stretches within the glass

Sodium ions and thus, creates stress on glass surface. During cooling, the potassium on surface shrinks little while the sodium in inner shrinks larger. Hence, stress is induced between glass surface and inside and consequently, the glass is strengthened.

Advanced process

There also exists a more advanced two-stage process for making chemically strengthened glass, in which the glass article is first immersed in a sodium nitrate bath at 450 °C, which enriches the surface with sodium ions. This leaves more sodium ions on the glass for the immersion in potassium nitrate to replace with potassium ions. In this way, the use of a sodium nitrate bath increases the potential for surface compression in the finished article.

Classifications

Chemical strengthened glass is classified by two strength components: surface compression and depth of layer (DOL). Surface compression values relate to flexural (bending) strength (MOR), impact strength, hardness penetration (scratching) and thermal shock resistance. Depth of layer values relate primarily to the amount of sustained abrasion resistance and the impact resistance of the surface compression layer.

Applications

Chemically strengthened glass was used for the aircraft canopy of some fighter aircraft. The chemically treated glass boasts a transparency range from the UV through the visible and into the infrared. This permits weapons systems designers to operate guidance devices whether they are CCD, radio frequency, infrared or laser based. The material's proponents stress that chemically treated glass is not just for use in military applications.

It can be used in numerous applications that demand toughness and optical clarity. The material is also useful for viewports, protective covers, and front surface optics in hostile environments whose elements may include high temperature, high pressure and vacuum conditions. Less demanding applications include point of sale scanner windows used in grocery store and retail scanners.

Fused Glass

Fusing glass in a kiln is a fascinating technique that enables artists to create unique and breathtaking designs in glass. Fused glass is also referred to as kiln-formed glass, art glass fusion and warm glass. The “warm” of warm glass is between 1,100 and 1,700 degrees Fahrenheit (600 and 925 degrees Celsius). At these temperatures, glass softens enough that when pieces of glass are heated and pressed together, they will fuse into a single seamless piece. This is the underlying principle behind glass fusing.

Glass Fusing

Glass fusing is the process of using a kiln to join together pieces of glass. If you apply heat to glass, it will soften. If you continue to apply heat, the glass will become more fluid and flow together. Two or more pieces of glass will stick to each other. When the right kind of glass is heated and then cooled properly, the resulting fused glass piece will be solid and unbroken.

Fused glass is normally fired (heat-processed) in a kiln at a range of high temperatures from 593 °C (1,099 °F) to 816 °C (1,501 °F). There are 3 main distinctions for temperature application and the resulting effect on the glass. They are as follows:

1. Slumping

2. Tack fusing

3. Full fusing

1. Slumping

Firing in the lower ranges of these temperatures 593–677 °C (1,099–1,251 °F) is called slumping. Slumping is a categorical description of an area of techniques for the formation of glass by applying heat to the point where the glass becomes plastic. The increasing fluidity of the glass with temperature causes the glass to 'slump' into the mould under the force of gravity. Glass is most commonly heated in an oven, often using glass in a sheet form and “slumping” it over a form or into a mould.

Moulds are generally made of high temperature plaster, clay coated with plaster or another release agent, graphite, sand mixed with a bonding agent, steel, or other materials. At the point where the glass has achieved the desired form the heat is quickly vented and the temperature reduced to prevent further movement of the glass and then it is stabilized at its respective annealing temperature and annealed.

2. Tack Fusing

Tack Fusing Glass refers to the effect that is obtained when two or more pieces of glass are heated to approximately 1350 to 1375 degrees F. This temperature range will result in any pieces of glass that are in contact with each other fusing together, while still allowing each piece to retain its' original shape, size and thickness.

3. Full Fusing

Several pieces of glass fused into a single finished piece of uniform thickness by heating them to somewhere between 1450 and 1475 degrees F is known as Full fusing. At these temperatures your glass will have melted enough to combine and flow together into a single piece of fused glass. This piece may be a finished piece or a starting point from which you cold work, cut or reshape the piece prior to another fusing.

Techniques

Most contemporary fusing methods involve stacking, or layering thin sheets of glass, often using different colors to create patterns or simple images. The stack is then placed inside the kiln (which is almost always electric, but can be heated by gas or wood) and then heated through a series of ramps (rapid heating cycles) and soaks (holding the temperature at a specific point) until the separate pieces begin to bond together. The longer the kiln is held at the maximum temperature the more thoroughly the stack will fuse, eventually softening and rounding the edges of the original shape.

Once the desired effect has been achieved at the maximum desired temperature, the kiln temperature will be brought down quickly through the temperature range of 815 °C (1,499 °F) to 573 °C (1,063 °F) in order to avoid devitrification. It is then allowed to cool slowly over a specified time, soaking at specified temperature ranges which are essential to the annealing process. This prevents uneven cooling and breakage and produces a strong finished product. This cooling takes place normally for a period of 10–12 hours in 3 stages.

The first stage- the rapid cool period is meant to place the glass into the upper end of the annealing range 516 °C (961 °F). The second stage- the anneal soak at 516 °C (961 °F) is meant to equalize the temperature at the core and the surface of the glass at 516 °C (961 °F) relieving the stress between those areas. The last stage, once all areas have had time to reach a consistent temperature, is the final journey to room temperature. The kiln is slowly brought down over the course of 2 hours to 371 °C (700 °F), soaked for 2 hours at 371 °C (700 °F), down again to 260 °C (500 °F) which ends the firing schedule. The glass will remain in the unopened kiln until the pyrometer reads room temperature.

Tiffany glass

History of Tiffany Glass

Tiffany glass is the generic name used to describe the many and varied types of glass developed and produced by Louis Comfort Tiffany, (1848-1933), one of the most famous stained glass artists of the United States; he was remembered not only for his windows but for decorative glass objects as well, in particular the so-called Tiffany lamps.

Tiffany was an interior designer, and in 1878 his interest turned towards the creation of stained glass, when he opened his own studio and glass foundry because he was unable to find the types of glass that he desired in interior decoration.

Tiffany Glass

Most people think of Tiffany glass as decorative bronze lamps with intricate multicolored, stained-glass shades, but it actually includes other glass products, including solid color windows, painted art glass shades and lamps, and flat and pressed glass. Tiffany glass pieces were incorporated into homes, most notably in lamp and window construction. The glass work was used in the homes of the wealthy, but also in public buildings.

Tiffany glass not only incorporates the color into the glass, but also tonal variations and texture, as well as use tonal variations to suggest depth. The pieces of glass were not evenly colored but were pieces of opalescent window glass made by combining and manipulating several colors to create an unprecedented range of hues and three-dimensional effects. Thus the tiffany windows look like paintings, which were therefore in great demand.

The Preston Bradley Hall dome put in place in Chicago's first public library in 1897 features more than 1,000 square feet of Tiffany glass. (Preston Bradley Hall is now home to the Chicago Cultural Center.)

Types of Tiffany glass

1. Opalescent glass

Opalescent glass is commonly used to describe glass where more than one color is present, being fused during the manufacture, as against flashed glass in which two colors may be laminated, or silver stained glass where a solution of silver nitrate is superficially applied, turning red glass to orange and blue glass to green. Some opalescent glass was used by several stained glass studios in England.

Opalescent glass is made with a combination of white glass and a cathedral color. The opacity of this type of glass is in relation to the amount of white glass used in its creation. Dense opal base glass uses a higher consistency of white glass than light opal base glass. Because of this change in mixtures, dense opal base glass is much more opaque than light opal base glasses.

Opalescent glass radiates especially deep, vibrant hues to achieve pictorial effects of unsurpassed beauty. This stunning stained glass piece features transparent enamels, silk-screened and kiln-fired on hand-rolled glass.

Opalescent glass is made in a number of ways, including as a single colour; with the pigments that give the glass a streaky, mottled, or cloudy appearance; and with or without a surface texture. It can be both a most beautiful and challenging glass with which to work. This is because the pigments are mixed into opalescent glass by hand during manufacture, with the result that the color patterns and tones in the glass are never exactly the same in any two sheets.

Opalescent glass has one characteristic that transparent glass does not: namely, that it can be seen in both transmitted and reflected light. Opalescent glass has color impregnated into it to the extent that the pigmentation is visible by light rays reflecting off it. It can be seen as well as seen through.

2. Favrile Glass

Favrile glass often has a distinctive characteristic that is common in some glass from Classical antiquity: it possesses a superficial iridescence. This iridescence causes the surface to shimmer, but also causes a degree of opacity. This iridescent effect of the glass was obtained by mixing different colors of glass together while hot. Favrile is different from other iridescent glasses because its color is not just on the surface, but imbedded in the glass.

Some of the distinguishing colors in Favrile glass includes "Gold Lustre", Samian Red"," Mazarin Blue", "Tel-al-amana" (or Turquoise Blue), and Aquamarine. Favrile was the first art glass to be used in stained-glass windows, as Tiffany first thought of the idea of making patterns in windows based shapes and colors.

3. Streamer Glass

Streamer glass refers to a sheet of glass with a pattern of glass strings affixed to its surface. Tiffany made use of such textured glass to represent, for example, twigs, branches and grass.

Streamers are prepared from very hot molten glass, gathered at the end of a punty (pontil) that is rapidly swung back and forth and stretched into long, thin strings that rapidly cool and harden. These hand-stretched streamers are pressed on the molten surface of sheet glass during the rolling process, and become permanently fused.

4. Fracture Glass

Fracture glass refers to a sheet of glass with a pattern of irregularly shaped, thin glass wafers affixed to its surface. Fracture glass is made from paper-thin blown shards or flakes of intensely colored glass fused to the bottom of sheets during the rolling process. Tiffany made use of such textured glass to represent, for example, foliage seen from a distance.

The irregular glass wafers, called fractures, are prepared from very hot, colored molten glass, gathered at the end of a blowpipe. A large bubble is forcefully blown until the walls of the bubble rapidly stretch, cool and harden. The resulting glass bubble has paper-thin walls and is immediately shattered into shards. These hand blown shards are pressed on the surface of the molten glass sheet during the rolling process, to which they become permanently fused.

5. Fracture-streamer Glass

Fracture-Streamer glass is fracture glass combined with hand-stretched streamers or strings of glass during the rolling process. Fracture-streamer glass refers to a sheet of glass with a pattern of glass strings, and irregularly shaped, thin glass wafers, affixed to its surface. Tiffany made use of such textured glass to represent, for example, twigs, branches and grass, and distant foliage.

The “fractures” are created by the addition of thin blown flakes of intensely colored glass, while the “streamers” are pulled or drawn strings of intense colors. Both fractures and streamers are quick-fused to the bottom of sheets during the rolling process.

Fracture and streamer glass is used primarily for backgrounds; the fractures suggest multitudinous leaves or flowers in the distance, while the streamers suggest twigs or stems. For this reason, fracture colors are usually selected to correspond to the colors used in leaf or flower foregrounds.

6. Ripple Glass

Ripple glass refers to a sheet of textured glass with marked surface waves. The texture is created during the glass sheet-forming process. A sheet is formed from molten glass with a roller that spins on it, while travelling forward. Normally the roller spins at the same speed as its own forward motion, and the resulting sheet has a smooth surface. In the manufacture of rippled glass, the roller spins faster than its own forward motion. The rippled effect is retained as the glass cools.

In order to cut ripple glass, the sheet may be scored on the smoother side with a carbide glass cutter, and broken at the score line with breaker-grozier pliers.

7. Ring Mottle Glass

Ring mottle glass is an opalescent glass in which rates of crystal growth have been controlled to create ring-shaped areas of opacity. The effect is a visual surface mottling. Ring mottle glass refers to sheet glass with a pronounced mottle created by localized, heat-treated opacification and crystal-growth dynamics. Tiffany's distinctive style exploited glass containing a variety of motifs such as those found in ring mottle glass, and he relied minimally on painted details.

This type of glass has a locally varying opacity; the “rings” are more opaque than the surrounding matrix. Ring mottled glass is used to provide color and image gradation that is non-streaky, or non-linear. The naturally rounded shape of each ring breaks up the more typical streakiness of stained glass. The artist, using ring mottles, can create shading and imagery unavailable from other glass types.

8. Drapery Glass

Glass sheets with multiple dramatic folds, likened to those in hanging drapes. Drapery glass refers to a sheet of heavily folded glass that suggests fabric folds. Tiffany made abundant use of drapery glass in ecclesiastical stained glass windows to add a 3-dimensional effect to flowing robes and angel wings, and to imitate the natural coarseness of magnolia petals.

To create drapery glass, the molten glass is shaped by taking a hand held roller and using it like a rolling pin to create "speed bumps" on the surface. It can also be tugged and pulled by hand using steel tongs to create the deep fabric-like folds in the surface. It is easy for the glassmakers to get burnt while making this unusual glass and extreme care must be taken while rolling the glass.