Swimming pool and spa expert witness, Paolo Benedetti discusses cracked glass tile and why it is not always the fault of the installer or installation practices.
Material Defects
Tessellated
stresses and microstresses (cracks, cracking and stress fissures) in
glass tiles and mosaics that contain recycled glass as a portion of it's
formulation, are problems that many tile mosaic manufacturers simply do
not understand or want to admit. Most are not even willing to subject their tiles to
any thermal shock performance testing. They bury their heads in the
silica... hoping that the problem will go away. It 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 and 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 and/or 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 the material science and physics of glass.
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 and performance standards upon these manufacturers.
Lack of Quality Control and 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.
Inconsistent Temperatures
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. The thicker the tile, the greater the problem.
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 and
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 (borosilicate glass, aka Pyrex) 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 and 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
Flint 95-100%
Amber 0-5%
Green 0-1%
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
seller's agreement.
Contaminant Listings:
Outthrow Materials: Organic Matter, allowable percentage
based upon buyer and seller's agreement.
Prohibitive Materials:
Ferrous Metals
Nonferrous Metals
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.)"
Unpredictable Results
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, movement, physical trauma or thinset shrinkage.
This is not to say that tesselated stresses can not coincidentally
align across 3 tiles. 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 opaque 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 and 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!
Larger format tiles are also subject to greater stresses from thinset shrinkage. The larger area creates greater tension on a large format tile verses a small mosaic. This has been demonstrated in a number of high quality thinsets.
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 and 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 and 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 and 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 and Material Sciences
These are not my hypothesis or mere suppositions... something that I made up.
These
are facts based upon over hundreds of years of the investigation
of physics and 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
M.I.T.
The American Ceramic Society
SciGlass
International Symposium on Glass Problems and countless foreign entities.
The Lack of Standards and 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 and various manufacturers. At least they'd have testing data for
compatible setting materials.
Performance standards for glass tiles are currently being developed.
And by whom? A committee of the manufacturers!
Paolo Benedetti
- Aquatic Artist, Consultant & Construction Defect Expert Witness
"Creating water as art."™
Aquatic Technology Pool and Spa
©www.aquatictechnology.com
This installation on the exterior of a round spa cracked due to the lack of movement joints (expansion joints). As the glass tile heats up in the sun, they expand. The rectangular shape of these tiles will cause them to expand more in the longitudinal direction. This creates a lot of stress.