Pilkington Optiwhite 559ºC/1039ºF 720°C/1328°F
Pilkington Optifloat 548ºC/1019ºF 720°C/1328°F
USA float (typical) 548ºC/1019ºF 615°C/1139
Information on stained glass, fusing, kilnforming and glass working
Pilkington Optiwhite 559ºC/1039ºF 720°C/1328°F
Pilkington Optifloat 548ºC/1019ºF 720°C/1328°F
USA float (typical) 548ºC/1019ºF 615°C/1139
There is a lot of imprecise terminology for refractory fibre paper and blanket. My interpretation:
Shelf paper is a very thin - like cartridge paper - material held together with organic binders, and often containing fibreglass particles. Thinfire and Papyros are two brand names.
Refractory fibre blanket. photo credit Amazon |
An example of a lead testing kit from Amazon |
There is legitimate concern about lead content of some glass intendended for culinary use. Surface lead testing kits have become popular and indicate the presence of lead on many glasses. It seemed to me that some evaluation of home lead test kits was in order. I looked at some sites for scientific evaluations and some reviews of testing kits and found these results.
Public Lab, whose mission is “Pursuing environmental justice through community science and open technology”, reports in the paper, “Evaluating Low-cost Lead Screening Products”, by Read Holman that “There are two evaluated [surface lead] test kits, the remaining three for surfaces have not been scientifically evaluated.” The report states that the tests for
“Paint/Surfaces...
There
are seven other scientifically evaluated tests for dust and water, which are
not applicable to glass surfaces.
The
conclusion of a report for the US Dept of Commerce states:
“Currently
available spot test kits cannot be used to determine lead-based paint, which is
defined as a paint having lead at levels equal to, or greater than, 1mg/cm2
[the allowable level]. This finding was consistent with conclusions from
several previously published field studies. As was found in the field studies,
the spot test kits in this controlled laboratory study generally gave
relatively high percents of false positives at the lead-based paint level of 1
mg/cm2. That is, the spot test kits were generally sensitive to lead
in paint at much lower levels” (p61)
The
experience of people using these tests (reviews on Amazon) show that almost all
surfaces show traces of lead, but at much lower concentrations than the allowable
levels.
A sample
review:
“We got a heart attack because what we wanted to test turned positive, we proceeded to then test other stuff as a control, and guess what? All positive. We got suspicious and started testing random objects that couldn’t possibly contain lead. They also turned positive!” JSP Lead Test Kit
The
high levels of false positives (up to 98%) leads me to question their value or
accuracy. Although I am not going to
spend money on any of these tests, I suspect the test kits will show lead on
clear glass too.
My
conclusion is that these tests are not reliable indicators of risky levels of
lead presence on the surface of glass artifacts. Any concern needs a much more reliable test
than the currently available surface lead test kits.
“Viscosity is not always lower for transparent glasses than for opalescent glasses. Opalescent glasses will begin to move more at temperatures of 538ºC/1000ºF than will transparent glasses, and even at 677ºC/1250ºF, there are still some opalescent glasses that move more than many transparent glasses. It is only when we get to fusing temperatures that we begin to see the majority of transparent glasses moving more than the majority of opalescent glasses. In general, it is correct that darker glasses will move more than lighter glasses – both because of their chemistries and because of their greater propensity to absorb infrared energy.”
Credit: Bullseye Glass Co. |
Annealing float glass seems take a long time. The annealing point (Tg) is higher than most fusing glasses, although float glass is part of the family of soda lime glass. This group of glasses should be cooled slowly from annealing temperature to 427ºC/800ºF and below to reduce risks of thermal shock. This makes a greater temperature range over which to anneal float than fusing glasses, consequently it extends the cooling time and increases energy expenses.
It does not have to be this way. Annealing of glass takes place over a range. This range extends below the published annealing point (Tg). This is the temperature at which equalisation can most quickly take place, but it is not as energy efficient as starting in the lower range. Annealing points (Tg) vary between manufacturers, but these are some of them:
Pilkington Optiwhite 559ºC/1039ºF
Pilkington Optifloat 548ºC/1019ºF
USA float (typical) 548ºC/1019ºF
Australian float (average) 550ºC/1022ºF
The annealing range extends to a practical 38ºC/68ºF below the Tg temperature. Annealing at a lower temperature can be as effective at the lower portion of the range as at the Tg. Using a lower annealing soak temperature reduces the temperature range of the first cooling stage by as much as 38ºC/68ºF, and reduces the cooling time without increasing risks of breaking. It also creates a denser glass according to scientific research. Denser glass is arguably a stronger glass.
This means that the annealing of
float glass can take place at the following reduced temperatures:
Pilkington
Optiwhite 521ºC/971ºF
Pilkington
Optifloat 510ºC/900ºF
USA
float (typical) 510ºC/900ºF
Australian
float (average) 512ºC/954ºF
This reduces the first cooling stage for 12mm/0.5” Pilkington Optiwhite from 2 hours 24 minutes to 1 hour 43 minutes. Forty-one minutes may not seem much but in electricity costs is significant. Also using the Bullseye concept of a three stage cooling, further savings can be made. Their research shows the second cooling stage to 371ºC/700ºF can be increased by 1.8 times the first cooling rate, saving further time and energy. The chart which shows these rates is Annealing Thick Slabs - Celsius and - Fahrenheit.
More information on annealing is available in the ebook Annealing: Concepts, Principles and Practice
Annealing float glass at the lower part of the annealing
range reduces the time and cost of firings.
Credit: Techniglass.com |
A new grinder bit chips the glass excessively, especially with a coarse grit. It
can also be the result of a bare spot on the bit. You need distinguish between these states. Check the surface of the bit. If there are any small bare spots, the bit
needs to be replaced.
Credit: WWGrainger |
The best thing to do with a new coarse bit is to treat it with a dressing stone. This is a block of aluminium oxide which can remove high points on the bit, and clean up the spaces between the diamonds on the bit. It is relatively inexpensive to buy and lasts a long time. The dressing stone can be a brick, although it is not as efficient because it is much softer.
If the grinding bit still chips off too much glass from the edge, you need a finer grit. It will not take glass off as quickly as the coarse one, but it eliminates or reduces the chipping. The three common grades are: coarse, standard, and fine. It is a good idea to maintain a stock of the medium and fine grit grinder bits as replacements for worn ones.
Credit: JMbestglass.cn |
Using core drill bits needs a drill press. It keeps the drill bit steady and avoids breaking the core which plugs the hollow part of the bit.
Oscillating a core diamond drill bit is not the correct
procedure. Oscillating the bit creates two undesirable things.
Credit: Lawson-HIS |
There are generic drill presses available for holding
Dremel-type craft motors and hand-held drills. They are inexpensive and make
the drilling process so much more certain to regulate the pressure. It also makes
an easier start without skipping over the glass. They are so inexpensive that a
few holes without skipping will pay it.
Credit: Bhole ST1542 Pico Dril |
Drill speeds should be varied according to the size of the
hole being drilled. This is important with the high speed Dremel-type
motors. Larger holes need a slower speed
than smaller ones. The rim speed of a small bit is nearer the rpm of the drill
than a larger one, because the larger one travels a greater distance per
revolution than a small one. A listing of recommended speeds is given in this blog.
Hollow core diamond bits are of two types:
They seem to have different designations in North America.
Bits of the first type are longer lasting, and more
expensive. These can be “sharpened” with an aluminium oxide dressing stick to expose
new diamonds and maintain their effectiveness.
Credit: W W Grainger.com |
Bits of the second type wear quickly and should not be “sharpened”
with a dressing stone. The normal wearing away of the bonding material exposes the new
diamonds. Dressing them wears
away the diamonds that could be used in drilling.
Another advantage to core bits, is that a core drill grinds out much less glass from the hole than a solid drill
bit, so it takes less time to drill a hole.
One disadvantage, especially on core drills of 5mm and less, is that the core needs frequent cleaning out of the cores that get stuck inside the drill bit. To maintain efficient and effective drilling, the core needs to be poked out from the bit from the base toward the drilling surface. This applies whether water is being pumped through the core or not. Without clearing the core, more pressure must be used to continue drilling, resulting in larger break outs as the hole is completed, and more breaks of the complete piece.
credit: Scarva |
Rigidisers are colloidal solutions of silica or quartz with
a carrier of some form. It is
also available as a powder to mix with water according to the instructions.
Silica and quartz (sometimes referred to as flint) in dry
powdered form are a serious health risk.
Wear good respiratory protection and long sleeves and gloves against its skin irritant. Work outside with
the powdered form to keep the dust out of the studio. Clean clothing
immediately after working with the powdered form of rigidiser. Wearing gloves is a good idea whenever
working with rigidisers, as the wet form is also a strong skin irritant.
Mix up the powdered form as 1 part powder to 4 parts water, by volume. Do this masked and gloved, and outdoors if possible. If not, have a HEPA vacuum running next to your work area. Mix thoroughly and allow to slake for 24 hours. Then mix very well by hand or with a blender. Strain the mix to remove any clumps - they can be made into a paste and added to the main solution.
Liberally paint the solution onto the refractory fibre. Stir prior to use and frequently throughout the application to keep the silica/quartz in suspension. Depending on permanence, coat one or both sides of the paper/blanket/board. It is not necessary to soak the fibre completely. The object is to provide a hard surface. It does not need to be hard throughout.
Flat Board
It is best to apply rigidiser on both sides of refractory board. If rigidising both
sides, allow one side to air dry before turning over to coat the other side. By coating both sides, the warping from heating
on one side is reduced.
Cover the shape you are taking the mould from with an
impervious separator such as Vaseline or thin plastic film. Prepare the fibre blanket by coating both sides of
the fibre with the rigidiser. It does
not need to be completely soaked. Press
the fibre firmly into/onto the shape and especially into any depressions and
around any protrusions to be certain of a faithful replication.
Allow the refractory fibre to air dry. Or if needed quickly, you can kiln dry at 90˚C
– 110˚C / 194˚F – 258˚F for several hours. But only if the master mould can withstand the
heat. If not, demould only after the
fibre is dry and can hold its shape without the master. Be sure to remove the master mould from the fibre before
proceeding to heat cure.
When air dried, cure in the kiln by firing to 790˚C/1454˚F
for 20 minutes. Before firing, place the dry form on a
refractory fibre separator to avoid the silica/quartz sticking to the shelf. A
rapid rate straight to the top temperature is acceptable. After the soak, turn the kiln off, as the rigidised refractory material
is not subject to thermal shock.
Coat the hardened fibre in kiln wash, or cover with shelf paper or refractory fibre paper, to avoid glass sticking to the hardened board. The bare surface of the rigidised form is now coated in glass fibres and they will stick to the glass unless a separator is applied.
When used as a shelf, it is best to turn the board over after
a few dozen firings. This helps counteract the warping tendency that rigidised
boards have.