Material: Recycled
Glass Issue:
The colors of recycled container glass - clear, green and
amber - limit the applications into which
it can be recycled. Methods of recolorizing recycled container
glass can help to expand recycling market
options, especially in high-value applications
such as art glass. Best Practice: Recolorizing container
glass can be achieved by adding chemicals
to decolorize or neutralize base colors,
then introducing coloring agents to achieve
a new range of colors.
Starting with post-consumer glass
cullet, it is possible to produce a wide
range of translucent colors as well as colors
with controlled opacity.
This Best Practice describes the
results of a recolorization project.
For many applications in which glass
is remelted, workability and thermal compatibility
are also important.
Those issues, along with strategies
for identifying cullet sources and making
batch additions, are treated in the Using
Recycled Glass in Art Glass Applications
Best Practice. Container glass
is formulated mainly from soda ash, limestone,
and silica sand (see Chemistry of Container Glass Best Practice). Container manufacturers produce brown glass
with a sulfur-iron colloidal solution, and
vary it from dark amber to honey amber with
additional agents.
Green bottle glass contains small
amounts of chromium dissolved in the glass,
with other colorants.
Testing has shown that a range of
colors can be achieved from color-sorted
recycled container glass.
However, it is important to note
that the resultant colors vary widely depending
on the source of glass cullet, furnace conditions,
and the types of batch chemicals added. Consistency of all conditions and materials
is absolutely critical.
The table below summarizes some color
modifications that have been made to samples
of clear, green and brown cullet.
Highlights of modification
potential for common container colors:
·
Brown glass can be decolorized
with the addition of zinc oxide to produce
a nearly colorless glass with a blue/green
hue. Erbium
Oxide (Er2O3) and
manganese oxide may be added to balance
the faint blue/green hue. From this base, other colors can be produced
with some of the common oxide colorants,
such as cobalt, manganese, titanium, and
copper.
By introducing CuO with SnO2
under reducing conditions (adding SiC reducer
to the batch), a deep copper ruby was produced
in test melts.
By changing the time of heat treatment,
the color can be struck to varying degrees
of intensity.
·
Green glass can be altered
by oxidizing the melt, shifting the chrome
redox equilibrium toward Cr6+,
which shifts the hue of the glass from the
intense green to a yellow-green.
By adding 0.5% Mn2O3
to the oxidized melt, the hue of the glass
is neutralized, yielding a neutral smoky
gray glass.
Again, from this neutral base, other
colors can be produced with the common oxide
colorants.
Densely colored, almost black, glasses
can be achieved using green glass cullet. Reagent grade or purer chemicals are best for optimal
color modification.
In the case of rare-earth oxides,
costs may dictate choosing lower purity
commercially available chemicals. Fluxing Agents can be used to facilitate melting base
glasses.
Two alkali oxides were used as fluxing
agents, added as water glass, 40° Be' solution
of Na2O*3SiO2, and
Potash (K2C03).
Boric oxide (B203)
was added as boric acid (H3B03). Zinc and tin may be added as their oxides,
zinc oxide (ZnO), and tin dioxide (SnO2). Colorants which may be added for decolorization or
recolorization of base glasses include Copper
added as copper (II) carbonate-dihydroxide,
cobalt added as cobalt carbonate, manganese
introduced as potassium permanganate, erbium
oxide, neodymium oxide, cerium dioxide,
and titanium dioxide.
Gold may also be introduced in the
form of auric chloride. Flourine-base opacifying agents have been used in test
melts to obtain opal colors.
Phosphorus and fluorine may be added
as phosphoric acid (H3PO4)
and calcium fluoride (CaF2).
A high fluorine-content salt called
PAF (Al3K6F9),
a by-product of aluminum refining, may also
be used.
Opal
glasses generally possess high marketability and value. However, test trials failed to produce consistent
opal glasses.
Flourine-based opalizing agents proved
difficult to blend, and separated in the
melt. This problem was exacerbated by the fact that
the fluorine tended to volatilize during
the melting process. All of the chemicals
above require special handling for safety.
Adequate ventilation is critical.
Consult Materials Safety Data Sheets for details. Fluorine volatility represents an especially
serious health
hazard, if not appropriately managed. Water, either chemical or liquid in the batch
materials, may increase the tendency of
flourine to volatize. Batch adjustments
should be made to balance the basic/acidic
glass melt using post-consumer bottle glass. Implementation:
This information
will be of use primarily to those already
skilled in hot glass. Benefits: The costs of raw materials for art
glass production are significant.
The information in this best practice
represents potential ways that recolorization
of common container glasses might be accomplished,
possibly saving money for studio operators
and increasing value for recyclers. Application Sites: Glass-blowing studios
and hot shops Contact: For more information about this Best Practice,
contact CWC, mailto:info@cwc.org. References:
Color Modification
of Post-Consumer Glass Cullet, Clean Washington Center, 1996. Dalbey,
Randal, Recycled GlassWorks, Seattle, WA
Issue Date / Update: November 1996
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