theglassguy
Harmless
Posts: 27
Registered: 11-10-2014
Member Is Offline
Mood: No Mood
|
|
Aluminum oxide/ "calcined" alumina- safe to melt into borosillicate glass in an oxygen/propane flame?
So I recently graduated with an associates in scientific glassblowing. While I did take a basic chemistry for scientific glass blowers class, I
definitely wouldn't say I'm a chemist. I attended the American scientific glass blowers society symposium last year, and attended a seminar on
borosilicate color. During the seminar they demonstrated mixing alumina oxide in the flame. Though I can't recall what grade of alumina oxide they
suggested. I have access to "calcined" alumina/aluminum oxide, but I'm not sure if this is the right/safe grade to use.
Could some of you actual chemists tell me if "calcined" alumina/aluminum oxide can be safely mixed with borosilicate glass in an oxygen/propane flame
without creating some kind of deadly off-gas. (Assuming I have proper ventilation of course)
|
|
|
violet sin
International Hazard
Posts: 1475
Registered: 2-9-2012
Location: Daydreaming of uraninite...
Member Is Offline
Mood: Good
|
|
the only danger I could see, would be if it were a fine powder that some how crystalized in sharp forms after exiting the flame... like breathing in
diatomaceous earth. I wouldn't think there are any chemical dangers. but I am not a "real chemist". just took high school and college classes years
back.
could you elaborate a little more on the process? the only paper I found while looking quickly, was more about strength and not color as far as I
read( http://www.gruppofrattura.it/ocs/index.php/ICF/ICF12/paper/v... ) I would assume from my limited glassblowing knowledge, borosilicate art/pipes
years ago, that you mean to heat the surface of the glass to a soft/tacky point. then dust alumina into the torch flame to adhere to the surface much
like in fuming silver? would that be even close?
one thing seems clear, particle size would be really important for an even covering thickness. if it was taken into a surface, i would assume the
varying depth of alumina might promote cracking/chipping on temp cycling.
doesn't seem like this would cause coloration by it self; is this an attempt to alter the surface chemistry to accept standard dopants/fumes in a
similar but distinctly unique way? unless you were talking in an optical line of reasoning. which would be way out of my league.
any papers you can link to give a guy an idea? like I said, I don't see a chemical reaction happening at all, but hazardous particles perhaps. I
would need to be pointed in the right direction to look for info for ya
|
|
|
theglassguy
Harmless
Posts: 27
Registered: 11-10-2014
Member Is Offline
Mood: No Mood
|
|
Basically the way I was taught was to weld a heavy wall tube onto a rod (like a cup). Fill the small tube with alumina oxide, weld a rod onto the top
of the cup rim, and heat up the oxide vertically so none falls out once it's all melted, proceed to mix and stir the two rods welded on to add mass
and thin out the color. Keep mixing and stirring in the flame until the color is uniform. Should turn a nice north star Forrest green. Assuming you
use the right grade of alumina oxide...
|
|
|
theglassguy
Harmless
Posts: 27
Registered: 11-10-2014
Member Is Offline
Mood: No Mood
|
|
Another technique I've heard is to roll a hot rod into the oxide a few times like you would with fritt, and mix clear with that, and stir until you
have a uniform color.
|
|
|
blogfast25
International Hazard
Posts: 10562
Registered: 3-2-2008
Location: Neverland
Member Is Offline
Mood: No Mood
|
|
I can't see that happening unless you use a doped alumina grade. Both borosilicate glass and alumina are colourless. Even 'melting' the alumina (MP =
about 2500 C !!) would not lead to any chemical reactions leading to colour.
Alumina doped with Cr<sub>2</sub>O,sub>3</sub> would give a green colour.
|
|
|
violet sin
International Hazard
Posts: 1475
Registered: 2-9-2012
Location: Daydreaming of uraninite...
Member Is Offline
Mood: Good
|
|
that's what I was thinking, blogfast25.
theglasguy, the alumina is possibly for the opaque nature of the northstar forest green,
( http://northstarglass.com/product/ns-53-forest-green/ ) NS-053:
- "A bright fully opaque green. Work in an oxidizing flame and heat slowly. It can boil if heated aggressively. See the Heat Sensitive color section"
- " For additional information about working NS-053 Forest Green, refer to Northstar Newsletter Issue 17. "
--> but the odd thing is that the link actually redirects here ( http://northstarglass.com/wordpress/wp-content/uploads/2013/... ) not at all what it says.
--> so instead read here: ( http://www.northstarglass.com/newsletters/17.pdf ) its the 17th newsletter referenced on the above page. found in the "downloads" section.
......" While chrome based colors are notoriously difficult to keep from cracking, especially in over-worked inside out applications, I have found
zero problems with this"
......" Any type of flame can be used, and neither time in the flame, over annealing, nor reduction, affects the chrome's compatibility. In fact,
eduction adds an appealing bluish hue on the color's surface if left uncovered. "
I actually remember using some of these colors well, I loved the amber purple, and a standard silver fume over clear boro, raked with a clear rod. I
seem to recall something about a fractional percent of something like copper or nickel to temper the chrome's shortcomings. the guy I was working for
had a knack for chatting people, distributors and suppliers. how much he absorbed,.. less than ideal.
he had a hard time dealing with the fact that he could talk the color subject to death on the others around, but I was "JUST" out of college chem
classes with everything totally fresh in my head... he really didn't understand the subject he was preaching. redox reactions were not his strong
point, so basic color shifts in flame types were a mystery. in the last class ( only chem 2) I had been able to calculate color of solutions observed
as a product of multiple equilibria systems with several ligands and metal salts in specific solvents. so basic colors were pretty easy. now I
barely know a *fraction of the foundation info, but some that I pursue because I find it interesting still lives. shoulda stayed in school... ughh
... never a genius, but not dumb either...
-------------
gotta say, I'm not trashing my past teacher, he was a very very skilled torch man. he just didn't know the chemistry of even the most familiar
colors, and talked as though he did. he sure as hell knew how to work the colors, varying the flame, and rarely burning out or shifting.
[Edited on 25-5-2015 by violet sin]
|
|
|
unionised
International Hazard
Posts: 5104
Registered: 1-11-2003
Location: UK
Member Is Offline
Mood: No Mood
|
|
Quote: Originally posted by blogfast25  |
I can't see that happening unless you use a doped alumina grade. Both borosilicate glass and alumina are colourless. Even 'melting' the alumina (MP =
about 2500 C !!) would not lead to any chemical reactions leading to colour.
Alumina doped with Cr<sub>2</sub>O,sub>3</sub> would give a green colour. |
Unless you get it hot enough that the alumina melts or dissolves (and using it as an opacifier suggests that you won't) the "green" colour associated
with chromium doped alumina is generally referred to as "ruby red"
|
|
|
violet sin
International Hazard
Posts: 1475
Registered: 2-9-2012
Location: Daydreaming of uraninite...
Member Is Offline
Mood: Good
|
|
wiki, ruby: http://en.wikipedia.org/wiki/Ruby
"Each Cr3+ is surrounded octahedrally by six O2− ions. This crystallographic arrangement strongly affects each Cr3+, resulting in light absorption
in the yellow-green region of the spectrum and thus in the red"
wiki, ceramic glaze: http://en.wikipedia.org/wiki/Ceramic_glaze
" Alumina, often derived from clay, stiffens the molten glaze to prevent it from running off the piece"
" sometimes opacifiers such as tin oxide or zirconium oxide, are used to modify the visual appearance of the fired glaze"
wiki, glass coloring: http://en.wikipedia.org/wiki/Glass_coloring_and_color_markin...
" Chromium is a very powerful colorizing agent, yielding dark green[6] or in higher concentrations even black color. Together with tin oxide and
arsenic it yields emerald green glass. Chromium aventurine, in which aventurescence was achieved by growth of large parallel chromium(III) oxide
plates during cooling, was also made from glass with added chromium oxide in amount above its solubility limit in glass"
wiki, tindioxide: http://en.wikipedia.org/wiki/Tin_dioxide
"boiling pt: 1,800–1,900 °C "
wiki, borosilicate: http://en.wikipedia.org/wiki/Borosilicate_glass
" Borosilicate is referred to as "hard glass" and has a higher melting point (approximately 3,000 degrees Fahrenheit / 1648 degrees Celsius
northstar, NS-053: http://northstarglass.com/product/ns-53-forest-green/
" A bright fully opaque green. Work in an oxidizing flame and heat slowly. It can boil if heated aggressively."
Heat Sensitive Colors: http://www.waleapparatus.com/client_images/catalog19836/page...
" There are many metals such as copper, cobalt, silver, and chrome that have relatively high boiling point and do not volatilize at low temperatures.
These are ideal colorants because once in the glassy state, they take aggressive heat without causing the glass to boil. There are some metal oxides
however that have extremely low boiling points that even when in the glassy state they do not take heat well."
---------------------------------------------------------------------------------------------------------
so, the alumina isn't the only thing encapsulating the chrome +3 like in ruby, its there for stiffness in lower amounts( I was guessing), and Cr +3 is
quite definitely known for green, and is intensified as well as opacified with tin dioxide. tin dioxide having a boiling point close enough to that
of the borosilicate to be boiled off if heated carelessly, is warned about by northstar specifically in reference to the the NS-053 green mentioned.
though the specific mixture of additives is proprietary, and not disclosed. it would be safe to say there are chrome III oxide, tin IV oxide, and
something else not mentioned ( possibly copper or nickle like my mentor said) to temper the glasses physical characteristics and colorfast nature
|
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
