A Discussion of Heat and Temperature and Their Application in Lampwork
Did you know
that before physics was named physics, it was called "Natural
Philosophy" ? That's no coincidence, and the term is appropriate. It is
the study of the laws of nature. Nowadays we tend to think of nature as plants,
animals and such, but all creation, including the properties and
behavior of glass, is subject to the laws of nature too. Indeed, nature is ubiquitous. It's physical laws are present and functioning in every aspect of life, from jet propulsion, to the water vapor whistling from aunt Millie's teapot.
While the concepts of heat and temperature may seem obvious, I
believe it's important to become perfectly clear on the definitions of,
and the difference between, heat and temperature, in order to take best
advantage of both when applying heat to the work. Trust
me, when you get a firm grasp on this and what follows here, either you
will discover new ways to accomplish your purpose more effectively, or
the ways you've already discovered will suddenly make more sense to you
and your skill level will indeed increase. If nothing else, if you are
like me you'll at least have the satisfaction of knowing why what you
are doing works or doesn't work. We have all had the experience of
making a beautiful bead, and then, frustratingly, not been able to
reproduce the results. Often the reason involves one aspect or another
of the application of heat. My goal here is to help you to be able to
understand and do just that, consistently and easily. (Well, at least
more consistently and easily.)
Okay, lets get started!
A mini physics lesson and definition of terms
As we begin to deal with the subject of heat and temperature, consider the following questions: Does a single matchstick
burn at the same temperature as the pine tree it
came from? Is the same amount of
heat present in both scenarios? Are two candles twice as "hot" as one?
How about 100 million candles? Depends on our definition of "hot"
doesn't it? We need to define a few terms here, if we're going to speak
clearly about this subject as it relates to lampwork.
So, think of heat as the quantity of energy
present in a given body, a bead, for instance. In the scientific sense,
it is the energy itself, and in that sense, it is "stuff".
Temperature is
a measure of a heated body's ability to transfer heat energy to
another body. Its magnitude, or intensity, if you will, is dependent upon, and a result of, the level of
excitation of the molecules in the heated body. In that sense, it is clearly not
"stuff". It is the "level of ability" of one thing to make another
thing "hot" by giving up some of its "stuff" to the other body. Get
this very clear: TEMPERATURE IS NOT A MEASURE OF HEAT.
Surprised? A swimming pool and a cup of water at the same temperature
contain vastly different amounts of heat. Their equal temperature tells
us that neither one can transfer any of its heat to the other.
We speak of the heat capacity
of a body as the amount of heat required to raise its temperature by a
specified amount. Heat capacity depends on the body's size, as well as the material comprising the body.
A swimming pool has an enormously greater heat capacity than a cup of
water. A pool full of kangaroos, Elvis impersonators, and drunk monkeys
will have a different heat capacity than a pool full of water.*
(*Boing! Thank you very much. Burp.)
Pop Quiz:
Now we can state clearly that the heat generated by 100 million candles is 100 million times greater than the heat of one candle, and also that the temperature
of the air above each of the 100 million candles is equal to the
temperature of the air above any one candle. As for the often
confusing, and unscientific term "hot", we may sometimes mean
temperature, heat, or just OW! In that case, it's both. : ^ (
Heat Transfer
There are three modes of heat transfer. They are:
Conduction: The transfer of heat energy from one body, or
reservoir, to another, by direct contact. An example of this is the
cooling of a bead as it is marvered, with the attendant heating of the
marvering pad. Another example is when you grab the wrong end of a
glass rod. Ouch! Yes, the rod's temperature was lowered by transferring
heat to your pinkies. And you became painfully aware of the other side
of the coin.
Convection: The transfer of heat to a fluid or gaseous medium in
circulation, and in contact with, a heated body. This is very similar
to conduction. Heat is carried away by contact with moving air or
water. An example of this is the loss of heat through hot air rising
out of an open kiln door, which is replaced by cooler room air entering
the kiln.
Radiation: The transfer of heat due to radiation (In our case,
light.) Radiation is a special case and differs from the first
two transfer methods, because it requires no physical contact or medium.
All bodies emit and absorb radiation continuously. If more is being
absorbed that emitted, there is a net increase in temperature, and
visa versa. Cooler bodies emit infra red (IR) light which is a "color"
of light that is invisible to the eye. It is truly light, however.
Warmer (hotter) bodies emit visible light, as in a glowing orange bead.
Still hotter bodies emit ultra violet (UV) light, also invisible to the
eye, but which can be very hazardous, and can burn the retina, as well
as causing damage to exposed tissues, including cancer. Thus the need
for using special protective lenses when working at high temperatures
with borosilicate glass. Some in the field recommend wearing sun block
lotion when working borosilicate, to protect exposed skin. The color of
an object determines which radiated wavelengths it can absorb and
convert to heat. Red absorbs all colors except red. Blue absorbs all
colors except blue, etc. Black absorbs all visible wavelengths, while
white reflects them all. Any color can still absorb heat by the first
two methods as well.
Implications of the definitions
As we add heat to a bead, the molecules become excited, and at some
point, we get a transition to a liquid state. Just when that transition occurs
depends, for one thing, on how big our bead is, or its heat capacity. A larger bead will
require more heat to be applied to attain the same temperature as a
smaller bead. We can accomplish that by either increasing the
rate at which we apply the heat (turning up the torch, moving into the
flame closer to the cone, etc.) or, simply applying heat at the same
rate (holding it at the same place with the same torch setting) for a
longer period of time.
As you are aware, a larger bead takes longer to cool as well as
longer to heat. The greater
the heat capacity of a body, the less the rate of temperature loss, as
well as gain. Your bead may be hard on the outside, but remain liquid
or partially liquid on the inside, or visa versa. This becomes an
important issue when making bicones, for instance. we'll discuss that at length later.
An important aspect
of glass is that it doesn't transfer heat very well by conduction, so it
takes longer than you might think for the heat energy to get out of the
center of a bead through the surrounding glass. Incidentally, the stainless steel
mandrel itself is also a relatively poor conductor of heat. Believe it
or not, it is almost 40 times less conductive of heat than copper.
That's why copper is used in heating and refrigeration coils, and why
your mandrel can be heated to 1500 degrees at one end, while your hand
at the other end is nice and comfy at 100 degrees or so. Don't
try using copper as a mandrel. I promise it's not a good thing! So,
see, our little physics discussion is paying off already.
I'm sure most of us have noticed the extra heat needed to melt
clear glass. A great deal of the radiated energy from the torch passes
right through the glass with no effect. We must rely for the most part on
conduction of heat by direct contact with the hot gases of the flame to
do the heating. One thing you may or may not have noticed, is that
clear glass will also cool more rapidly, for the same reason. That is
because emitted radiation from the center of the bead can pass out
directly through the glass, since there is no opaque, colored material
to reabsorb it. The same thing applies to white, and some other colors
that change from transparent to opaque depending on temperature. In the
transparent stage, cooling by radiation can proceed much more rapidly
than after the change to opaque.
Applying stringer:
High temperature, low heat would be my choice. Yes, that's the hi O2,
low fuel, needle fine flame you're likely already using. It's just right for
melting the low heat capacity stringer, while hardly affecting the higher
heat capacity base bead.
Since part of the means by which glass is heated involves
radiation from the torch, white and clear glass will absorb heat more
slowly than darker colors. Consider the implications of this, and the
foregoing note on stringer and heat capacity when applying white
stringer to a black bead vs. applying black stringer to a white bead,
and certainly when working with clear stringer and / or encasing. You
may well melt dark opaque details on the bead before the clear flows,
or more perhaps more likely, right through the newly applied encasing,
since now, you've heated the underlying opaque by conduction, and just
a smidgen more heat from the torch may melt it by radiation, even if
the clear has mostly solidified.
It's a differential equation
As stated earlier, all bodies lose heat by
radiation constantly, even while absorbing it by any of the three
methods. We can have a net gain or loss of heat depending on the rate
of the gains vs the losses. It's like filling a leaky bucket, or
bailing a leaky boat. The rate of heat loss is proportional to the
surface area of the bead. i.e., The more surface area there is, the more
air contact, and the larger the area that is radiating. To
complicate matters, as the thickness increases, it is harder for the
heat to reach the center. For that reason, the heat loss from a large
bead will at some point equal the heating capacity of a given torch.
Hot Head owners are some of the first to discover this aspect of
beadmaking physics. I have also hit that ceiling on my minor burner. I
once succeeded in making a bead the size of a small egg, only to have
it crack before I could get it into the kiln. The first sign you're in
trouble may be the inability to keep the entire bead soft enough to
shape it. It will cool from the back, while you're heating it from the
front. Rotating it will not help, because then you are also allowing
less time for heat absorption in the front. As the ability to heat the
center decreases, but heat loss from the center continues, (though
decreasing as well,) A larger and larger temperature difference
develops between the inner glass and the outer. We all know the result.
A second, more common problem along these lines comes into play with
trying to make large beads without an annealing kiln. Since the surface
area is large, the outer layer is cooling more rapidly, while the large
volume tends to slow the cooling rate of the center. Larger thermal
stress is again the result.
COE: Coefficient of Expansion
I would be remiss I guess, not to discuss this subject here, since
it is of universal concern to lampworkers. However, as long as we stick
to all one type or brand of glass in a given bead we're pretty safe.
Most of us are at least familiar with the term, and very many of us
understand the meaning as relates to the physics. For those that would
like some info on this, while I can't provide an authoritative discussion on it, I can perhaps shed a little light.
Coefficient: In mathematical terms, simply a multiplier.
Expansion: Just that. Getting bigger.
A property of all solids, liquids, and gasses is that in general,
when heated they expand, and when cooled, they contract. For most
solids, the change in size is quite small. This change in size is
proportional to a change in the temperature of the solid, and
visa versa. A good example of this is in your toaster on your kitchen
table. Inside there is something called a bimetal strip. It is a thin
sandwich of two different metals, such as copper and steel, for
instance. They are bonded together over the entire length of the strip.
As the heat from the toaster warms the strip, it bends, due to the
differing COE of the metals. When the strip bends far enough, it
releases a catch, and your toast is burned as usual. Can't someone make
a darn toaster that works?
Glass is no exception to this property of matter, and unlike copper and
steel it does not like to bend! The molecular forces in the glass can
be extreme, leading to shattering.
COE is expressed, in general, in units of (amount of expansion)
per (length at some standard temperature) per degree temperature rise
(usually centigrade). 90 COE glass is shorthand for 90 * 10 to
the -7th power, or .000009 cm/cm/deg C (See, isn't it easier to just
call it 90?) Me too.
I will note here, that equal COE does not necessarily imply
compatibility, nor does compatibility always imply equal COE. There is
much information on this issue, and much misinformation as well, on the
web, so I leave it to you to investigate if you want to delve
further. Bullseye.com has a good article on the subject, and I've seen
many others.
Next time:
Perhaps the most important aspect in forming lampwork
beads is having the proper amount of heat in your bead for a given
operation.
Actually, it's the proper amount of heat and it's location that is at issue. We'll discuss that in an upcoming article. Meanwhile, happy beading, and enjoy the magic of glass!
This article may be copied for personal use, provided it remains
complete and intact and in unaltered form, including this notice.
Publishing on the web, or by any other means is prohibited without the
express permission of the author.
(c) 2004 David Fousek / ArtintheRound.com
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