Glass Tile Mosaics - Tessellated Stress
By Paolo Benedetti, Tuesday, June 15, 2010
Glass tile mosaics - tessellated stress cracks fissures microstresses
Recycled glass: Bad for glass tile, but good for the environment (and manufacturer's profits)!
Tessellated stresses and microstresses (cracking & stress fissures) within glass tile mosaics that contain recycled glass as a portion of it's formulation, are problems that many tile mosaic manufacturers simply do not understand. Most are not even willing to subject their tiles to any thermal shock performance testing. They bury their heads into the sand.. hoping that the problem will go away. It won't... (and I won't!).
If the manufacturer's never test their tiles for thermal expansion stress cracking, then they can continue to "play dumb." This allows them to "blame" the installation, because they know that it will be difficult & costly for an installer or property owner prove that there are materials or manufacturing issues. However, the underlying problem actual lies in their manufacturing process & the improper mixing of dissimilar materials.
With so many companies jumping on the environmentally "green" bandwagon, many are manufacturing products that are not durable. They simply do not understand material science and physics.
It will only take an Act of Congress, Consumer Product Safety Commission, a State Attorney General or a Class-Action Lawsuit to enforce minimum quality & performance standards upon these manufacturers.
Lack of Quality Control & Education
Almost all of the manufacturers fail to understand the importance of a homogeneous mix of virgin materials with the recycled materials. The recycled materials must be mixed thoroughly with themselves and with any additives or raw materials that are added.
If there are portions of the batch that are not homogeneous, there will exist a differential blend of material throughout the finished product. The laws of Physics dictate that different materials have different rates of thermal expansion.
Because these different materials expand at ever-so-slightly different rates, and because glass is not elastic, this stress will cause stress cracking or fissures.
Adding to the problem, is another law of physics. Thermal expansion occurs within matter at a rate in proportion to the temperature. In an installed application there exist temperature gradients throughout glass tile. The surface is heated or cooled at a rate that is faster than the back of the material.
Remember: the back of the material is usually mounted against a cool concrete structure. When the surface of the glass is exposed to the sunlight or swimming pool, it heats up at a faster rate than the back of the tile.
This "temperature gradient" can cause cracking within the tiles. Coupled with the use of recycled materials, and you have the formulation for failure.
Material Size and Thickness
We've established that there can be a temperature gradient within a singular tile. Physics also dictates that thicker tiles will have greater degrees of temperature gradient than thinner tiles, from top to bottom.
Stresses will be less likely to occur in smaller format tiles (1x1's) than in larger format tiles (3x3's, 4x4's, 6x6's +). Irregular sizes (rectangles, triangles, circles, etc.) can also create unusual stresses within the glass tile. This is not to say that they will not occur in small tiles - they do. Given that a small tile may crack, it is almost guaranteed that larger formats of the same tile will crack.
This occurs because the edges remain cooler than the center, creating gradient temperatures across a larger surface area. On a smaller tile, the gradient is less as the core is closer to the edges.
Thicker tiles also contribute to the gradient temperature issue, as thicker tiles cause "shading" of neighboring tiles, allowing the sides & edges to remain cooler than the core or surfaces.
Add coatings to the glass, and you have further altered the possible temperature gradients, by absorbing or reflecting heat in an irregular fashion.
Unknown Raw Materials
Just because a manufacturer receives all of their "raw recycled glass" from one source, does not automatically mean that it is all the same. If a manufacturer uses only recycled soda bottles from one brand of soda - there is no way on God's Green Earth that they can be 1000% certain that all of the bottles are EXACTLY the same.
The soda bottle manufacturer may change formulations ever so slightly from batch to batch, manufacturing temperatures may vary slightly, or even their raw minerals or suppliers may vary slightly.
Since most manufacturer's rely on the "say so" of their supplier to verify that all of the bottles are the same, there is another source of doubt as to the "quality" of the recycled glass. Because the recycled glass is already ground up (aka: "cullet"), there is no means to test the glass to verify the veracity of their claims. It could have soda bottles, old windows, automobile glass (argh!!!), beer bottles, food jars, or contain labeling contaminates such as cobalt (like is found on Corona bottles).
If there is the slightest piece of heat-resistant glass like Pyrex or borosilicate glass present in the cullet, it will alter the viscosity of the fluid in the furnace when it is remelted.
Add to the variance equation, the fact that there may be multiple suppliers of soda bottles to the bottler - each with their own formulation & raw material suppliers, and you have sufficient material variances to wreak havoc with the performance of glass mosaic tiles.
The recycled glass association's standards allow variances in the cullet mix, proof again that there are contaminates and variables in the "raw recycled glass." Here are their acceptable standards:
"PROCESSED (FURNACE READY) FLINT CONTAINER GLASS CULLET SPECIFICATIONS
Composition: Soda-lime-silica container glass.
Container Glass Cullet Colors Segregation: Flint Cullet
Other Colors 0-.5%
Total NON-Flint Cullet = <5%>Size: Various sizes from whole glass containers to -100 Mesh.
However, the ideal material size is 3/8" to 3/4" with a 10% minimum
of fine particles. Material size is based upon buyer and
Outthrow Materials: Organic Matter, allowable percentage
based upon buyer and seller's agreement.
Ceramics (such as cups, saucers, dinnerware, pottery, etc.)
Other Glass (for example, plate window glass, heat-resistant
glass-such as Pyrex-and lead-based glass-such as
crystal ware, television tubes, vision ware, etc.)
Other Materials (such as bricks, rocks, etc.)"
Most of the models for predicting the performance characteristics of glass formulation rely on multiple regression analysis or by additivity equations. But the primary principle for utilizing these mathematical predictions, is that you know the formulation of ALL of the raw materials - totally impossible when using recycled glass!
As with any quality control procedure, these equations are the same: garbage in, garbage out. Since they cannot possibly ascertain the chemical composition of 100% of the recycled glass cullet, they are forced to either "guesstimate" or use a random sample of the cullet (and assume that it is all the same!). This is where the variability begins... right on the loading dock!
Why so Random?
Because the materials are not thoroughly mixed, it reasons that there are areas of the sheet of glass, wherein there exists higher concentrations of heterogeneous material. Because the mixing may be more thorough in one batch than the next, concentrations throughout the glass may vary, and tiles may be mixed with other batches of the same color, the resultant cracking in tiles will appear to be totally random.
It may not occur in every color or size of the same tiles. It may appear in adjacent tiles or they may appear in isolated tiles. But there is often a major commonality... the cracking is not linear. Tesselated stresses may transfer into an adjacent tile.
However, if the cracks are directly in line with each other and through a minimum of 3 tiles, then thermal stresses are probably not the cause. Linear cracking through multiple (3+) tiles is probably the result of substrate flexing or movement. This is not to say that tesselated stresses can not coincidentally align across 3 tiles. But there is a statistical probability of this occurring in 2 adjacent tiles, so 3+ has become the rule of thumb.
If the cracks are not through the entire tile, then there is a high probability that it is not installation related. This is "clearly visible" on clear tiles - if they look like fissures within an ice cube, then they are probably thermal stress cracks.
Remember, the larger the format of tile, the lower the tolerance for substrate movement & the greater the chance of thermal stress cracking. The problem is compounding itself!! Example: A lot of grout joints in 1x1 mosaic tiles across an area are a lot more forgiving than rigid 12x12 tiles!
Getting it Mixed Right
There are a few approved methods to ensure that recycled glass is utilized correctly. However, it requires that the glass formulation is mixed thoroughly.
One method is to actually mix the molten glass with a mixer, something that is difficulty, costly & dangerous. Most manufacturers utilize linear kilns - a conveyor belt within a long furnace. This makes mixing molten glass in this method impossible.
The second method, involves multiple stages & involves grinding the glass multiple times. First the recycled glass is ground as small as possible (cullet). The finer it is ground, the more thoroughly the blend can be mixed. Virgin materials are mixed into the mixer along with the ground recycled glass. This mix is then fired into glass utilizing a linear kiln.
Unfortunately, this is where most manufacturer's end their processing. They form their tiles and ship them out the door.
To ensure that the various glass formulations are thoroughly blended, there are two more stages of processing required.
To ensure a thorough homogeneous blend, the glass that is made during the first stage above, is process again. It is broken up & ground up as fine as possible. This fine pulverized glass is again thoroughly mixed. Now it can be fired and made into consumer ready glass mosaic tiles.
Ensuring that there is a homogeneous mix entails additional processing and a double firing of the glass, which is costly. The production time, energy and labor costs more than double! Therefore, most manufacturers who utilize recycled glass merely skip this step.
Some manufacturer's add chemical that they claim solve the issue of homogeneous blending. However, it is statically impossible to prove that the end result will be a homogeneous and isotropic blend. And since there is a very high probability that the cullet is contaminated, there is no means to chemically treat for all possible variations - Who's fooling who???
Principals Founded in Physics & Material Sciences
These are not my hypothesis or mere suppositions... something that I made up.
These are facts based upon over 100 hundreds of years of the investigation of physics & material sciences. In fact, the phenomenon of glass stress cracking from non-homogeneous blending was discussed in scientific papers as far back as the 1890's! Many organizations have investigated this phenomenon:
National Institute of Standards and Technology
Corning Glass Works
The American Ceramic Society
International Symposium on Glass Problems
and countless foreign entities.
The Lack of Standards & Differential Expansion
The development of tessellated stresses in glass tiles that utilize recycled material will continue to occur, until manufacturers are held to some standard for thermal shock performance.
If they subject their tiles to the CTIOA (Ceramic Tile Institute of America) thermal shock testing (who's tests are designed for ceramic tiles - which do not apply to the performance testing of glass tiles), they only have to submit a mere 5 tiles for testing.
This is not a representative sample of the tile's performance. Again - simple statistical analysis...
Nor is allowing the testing of ONE size of each product line, representative of the entire products line's performance. Glass of different colors contain different chemicals and formulations, and glass tiles of different sizes perform differently. Given their choice, the manufacturer's will submit clear glass 1X1 tiles, who tests will then be proffered as representative of an entire product line.
Manufacturer's are not required to re-certify or submit subsequent production lots for verification of continued compliance and quality control. As the recycled glass (raw materials) change from day to day, so will the final product's performance - yet they will still be relying upon those initial test results.
The CTIOA testing also tests the tiles in their "loose" unmounted state. Now who buys tiles to throw them loose into a pool? They should be tested in their mounted condition, with approved setting materials. Yes, multiple variables & various manufacturers. At least they'd have testing data for compatible setting materials.
Currently there are no performance standards for glass tiles.
It truly is the...
"Wild Wild West!"