Glass Quench and Strain Marks
Strain Pattern or Quench Pattern Characteristics
Heat-treated glass (heat-strengthened or tempered) can have an optical phenomenon that is called strain pattern or quench pattern. This phenomenon can appear as faint spots, blotches, or lines. This is the result of the air quenching (cooling) of the glass when it is heat-treated and should not be considered a glass defect.
The strain pattern or quench pattern is a result of heating the glass to its softening point and quenching the glass with high velocity air. Heat treatment of glass is required when safety glazing is required (tempered), or to reduce thermal breakage potential or to improve windload resistance.
Due to the aesthetic considerations with heat-treated glass products, the design professional should consider the appropriateness of specifying, recommending and using heat-treated glass products.
The heat treatment process results in a higher surface compression directly opposite the air quench, air nozzles or slots. The higher compression areas are denser and can exhibit a darker appearance under some viewing conditions.
The American Society for Testing and Materials (ASTM) has a standard on heat-treated glass products. This standard is C1048-04 Heat Treated Flat Glass – Kind HS, Kind FT Coated and Uncoated Glass. In this standard, strain pattern is recognized and identifi ed as follows:
7.5 Strain Pattern – “In heat strengthened and fully tempered glass, a strain pattern, which is not normally visible, may become visible under certain lighting conditions. It is a characteristic of these kinds of glass and should not be mistaken as discoloration or non-uniform tint or color.”
In addition, the Glass Tempering Division of the Glass Association of North America defines Strain Pattern as follows:
“The tempering process places glass under very high compression on the surface and high tension in the core of the glass. This results in a specific geometric optical pattern in the glass which is not normally visible, but which may become apparent under certain conditions of illuminations, especially when light is polarized, such as a skylight or other forms of reflected light. The colors of the strain pattern are sometimes referred to as iridescent, or the general condition as iridescence. The pattern that is seen under certain lighting conditions may vary from manufacturer, depending on the design of the cooling apparatus. Strain pattern is characteristic of all fully tempered glass.”
The intensity of the quench or strain pattern is influenced by the viewing angle, lighting conditions and by the perceptiveness of the viewer. It is nearly impossible to eliminate the strain pattern or quench pattern in heat treated glass products.
The strain or quench pattern may be accentuated if two lites of heat-treated glass are used in an IG unit. These strain patterns or quench patterns should not be viewed as a defect in the glass as they cannot be eliminated in the heat treatment process. In addition, GANA has published the attached Glass Information Bulletin GANA TD-05-0108 on Quench patterns in Heat Treated Architectural Glass.
Glass information Bulletin GANA TD-05-0108
Quench Patterns in Heat-Treated Architectural Glass
Glass used in architecture today commonly includes clear and tinted glass substrates, low-emissivity and solar-control coatings, decorative ceramic-frit patterns and safety glazing considerations that require glass to be heat-treated. Heat-strengthened and fully tempered glass is designed to meet increased thermal and mechanical stresses, or other specified physical criteria. As a result of the heat-treating fabrication process, quench patterns/marks or what is often referred to as a “strain pattern,” may become visible in heat-strengthened and fully tempered glass under certain natural or polarized lighting conditions.
ASTM International consensus document C 1048 – Standard Specification for Heat-Treated Flat Glass – Kind HS, Kind FT Coated and Uncoated Glass addresses this optical phenomenon as follows:
Section 7. Fabrication
7.5 Strain Pattern – In heat-strengthened and fully tempered glass, a strain pattern, which is not normally visible, may become visible under certain light conditions. It is characteristic of these kinds of glasses and should not be mistaken as a discoloration of non-uniform tint or color.
The intensity of the strain pattern may vary from lite to lite, and/or within a given lite. The presence of a strain pattern or the perceivable differences in the strain pattern is not a glass defect or blemish and is not cause for rejection. In addition, the presence of a strain pattern does not alter the structural integrity or safety of the glass lite.
The Heat-Treating Process
In order to provide the required resistance to thermal and mechanical stresses, and to achieve specific break patterns for safety glazing applications, annealed float glass is strengthened through a thermal process known as heat-treating. Heat-treating includes both heat-strengthened and fully tempered glass. The most commonly used process for heat-treating architectural products requires glass to be cut to the desired size and shape, edges prepared to the specified condition, surfaces and edges to be thoroughly washed and for the lite of glass to be transported through an oven and uniformly heated to near its softening point of approximately 1150 °F (621 °C). Upon exiting the oven, the glass is rapidly cooled (quenched) by blowing air uniformly onto all surfaces simultaneously. The quenching/cooling process places the surfaces of the glass in a state of high compression and the central core in compensating tension.
The high velocity air of the quench process is applied through air nozzles or slots. Glass surfaces directly opposite the nozzles or slots achieve a slightly higher level of surface compression than adjacent areas. This creates a very slight change in density causing the glass to become optically anisotropic. Optically anisotropic means that the appearance of light passing through the glass differs as your eye moves across the glass. When polarized light from the sun passes through the heat-treated glass it experiences a phase shift. The areas of lower density (directly under the quench nozzles) have a different phase shift than the areas with higher density (away from the quench nozzles). These slight differences in density result in light and dark areas observed in the glass.
Pattern Visibility
While not normally visible, a pattern of perceived faint light and/or dark spots or lines in heat-treated glass may become apparent under certain light and viewing conditions. The “quench pattern” is most apparent under polarized light with a visible horizon and viewed at an oblique viewing angle to the glass surface. The visibility of the pattern decreases as the viewing angle to the surface of the glass increases. When viewing from the interior of the building, the quench pattern may be visible from a 10° viewing angle and not apparent at a 90° viewing angle from the surface of the glass. When viewing the glass in reflectance from the exterior of the building, the quench pattern may be visible when looking at the glass surface at a 30-60° angle. Visibility of the quench pattern may be accentuated with thicker glass, tinted glass substrates, coated glass and multiple lites of heat-treated glass in
laminated or insulating glass products. As a result of variations in fabrication systems (or tempering furnace systems), the quench pattern may vary from one fabricator to another.
As frequently seen in back and side lites of automobiles, the quench pattern in the fully tempered glass can become more visible when wearing polarized sunglasses. Polarizing filters or lens for cameras will create the same phenomena and may cause the pattern to become more visible.
Glass Inspection
Construction sites may yield viewing angles and conditions that cause the quench pattern to become visible. However, upon completion of construction; the presence of interior walls; finishes; furniture; and plants frequently results in the strain pattern being less visible or not visible at all.
The stresses introduced in the heat-treating of glass are an inherent part of the fabrication process, and while they may be affected or altered depending on the heating process, controls and/or quench design, they cannot be eliminated. Design professionals should be aware that quench patterns are not a defect in heat-treated glass and, therefore, are not a basis for product rejection.
The phenomenon of quench pattern may be visible in any heat-treated glass. While pre-, or early-construction applications will not provide final project conditions, consultation with the glass supplier and viewing full size mock-ups under typical site conditions and surrounding landscape may be helpful in evaluating the potential for visibility of the quench pattern.
This extract is taken from The Glass Association of North America (GANA) technical bulletin, which was produced solely to provide general information as to quench patterns in heat-treated architectural flat glass. The Bulletin does not purport to state that any one particular type heat-treating process or procedure should be used in all applications or even in any specific application. The user of this Bulletin has the responsibility to ensure the product literature from the heat-treated glass fabricator is considered in the selection and specification of the glass. GANA disclaims any responsibility for any specific results related to the use of this Bulletin, for any errors or omissions contained in the Bulletin, and for any liability for loss or damage of any kind arising out of the use of this Bulletin.
Optical Distortion
Heat strengthened glass is twice as strong as annealed glass of the same thickness. This added strength provides the opportunity to produce larger glass panes whilst meeting wind load requirements without the necessity of increasing the thickness of the glass, thus controlling the weight of the unit and the optical clarity.
However, heat strengthened glass has other properties caused through the manufacturing process, of roller wave, local bow, edge bow and kink.
Distortion, whilst detectable, is usually within acceptable limits. Distortion of images, whether viewed in transmission or reflectance may be accentuated when viewed at acute angles.
Brewsters Fringes
The appearance of the optical phenomenon known as Brewster’s Fringes is not a defect of the glass, and can occur with any glass of high optical and surface quality. This phenomenon is a result of the high quality now being achieved worldwide by modern methods of glass manufacture. Brewster’s Fringes occur if wavelengths of light meet up with each other when they are exactly 180º out of phase – an example of the phenomenon known to physicists as the interference of light.
The effect is similar to, although usually much smaller than the interference fringes which can sometimes be seen on toughened glass windscreens.
Interference fringes are a series of faint, irregular, roughly parallel lines, which are grey or colored, and are sometimes visible in clear insulating glass (IG) units. They are typically a fraction of an inch wide and many inches long. They are caused by interference effects when light waves reflect from parallel, or nearly parallel glass surfaces.
Interference fringes are caused by reflections between the various glass surfaces of an IG unit when one light wave is delayed relative to another. Fringes are seen because of the constructive or destructive interference which occurs when two light waves combine. If the waves are in phase the resulting light will be stronger and brighter. If the combining waves are in opposite phases they cancel each other and the resulting light is weak. The easiest fringes to see are the colorful Newton’s Rings which occur when the two glasses of an IG unit touch at the center. The contacting glass area will be surrounded by concentric curving fringes. Tapping on the glass usually has no effect on these fringes because the glass to glass contact prevents any change in the geometry of the reflecting surfaces.
These fringes are caused by the slight delay between two light waves reflected from the two surfaces on either side of the very narrow tapered air gap, resulting in
constructive or destructive interference.
Newton’s Rings can result from an IG manufacturer failing to control the sealed air space pressure when a unit is sealed, or when two very heavily dished tempered pieces of glass are fabricated with both concave sides facing outwards. Fabricating very large sealed units when the sides overhang the edges of a narrow conveyor can also bend the glass sufficiently to cause glass contact in the center. The glass in a properly made IG unit (i.e. both glass lights parallel to each other, at room
temperature, when the air space is sealed) will never touch at the center no matter what the unit size, air gap width (typically between ¼” (6mm) and 1” (25mm)), wind load, or atmospheric temperature and normal barometric variations experienced. Extreme changes in barometric pressure caused by altitude differences of more than a few thousand feet between sealing and installation locations may be enough to cause glass touching at the center. Newton’s Rings in an IG unit can often be cured by inflating the sealed unit through a temporary hole in the spacer and resealing the unit when the two glasses are parallel. Units should be inflated before the glass surfaces have had time to rub against each other, creating a visible mark or abrasion.
Another type of fringe pattern, sometimes called Brewster’s Fringes, is less easily seen but can occur in a sealed unit with two pieces of nearly identical glass thickness (“4 Surface Reflection”) or very rarely with two pieces of flat glass of different thickness (“3 Surface Reflection”). At a glass surface a light wave can be split, because of partial reflection, into two new waves. These new waves can travel different paths, of slightly different lengths, and then recombine after further reflections as shown in the two diagrams. A variation in glass thickness of only 0.00004” (0.0001 mm) can sufficiently delay one wave relative to its partner to cause fringe formation. Brewster’s Fringes often require special lighting and viewing conditions to be seen, such as viewing at an angle rather than directly at the glass and with a shaded area beyond the glass. Most people never see Brewster’s Fringes because their eyes are focused upon the exterior view which is considerably brighter and masks the fringes. To see fringes, it is necessary to look at the glass rather than through it, an unnatural condition for the untrained eye. Brewster’s Fringes have little color or are simply grey. When the glass surface is tapped or lightly pressed the fringes move in response to the small changes in the geometry of the surfaces.
There is no cure for Brewster’s Fringes. They can often be minimized by making IG units with glasses of two different thicknesses. Insulating glass manufacturers often use DS (3.0 mm) and DST (3.2 mm) glass in the same unit to prevent this condition. A thickness difference of at least 0.003” (0.08mm) is needed to stop ‘four surface’ Brewster’s Fringe formation. Often glasses cut from opposite sides of the same float ribbon can have just enough thickness difference to prevent fringes.
But two pieces of glass cut one after the other along the length of the float ribbon, and which preserve the same orientation during assembly, will have a greater chance of matching the small thickness variations and of creating Brewster’s Fringes. Fringes are sometimes seen because today’s float glass is so flat and free of distortion that it allows this optical phenomenon to be observed.
