Re: [CAD_CAM_EDM_DRO] flat glass wasRe: re:Levelling large table

> I have a minor understanding of order & entropy
> but could you give a lay persons intro to differential
> calorimeter? And it's application in the glass/liquid thing?
>
> -G
> (ALWAYS looking for more information)
>

Yes always looking for more information is a good thing!

I could not claim expert status on the subject either but I have
commissioned many DSC (differential scanning calorimetry)
analyses of materials in the past from my friendly local DSC
guru. Usually on amorphous or partially crystalline polymers.

I used this to develop some positive temperature coefficient
conductive polymers that were reasonably successful. They use crystal growth
and quantum mechanical tunneling to achieve this.

Now I hack on wood though! :^)

I have seen some good stuff on this in the hybrid ceramic
electronic industry.

Fine structure, Particularly crystalline structure, is a lower energy, lower
entropy state. Nature tends to seek the lowest
energy state so glasses are in a non equilibrium state. One has to
differentiate between glass as a material description
vs glass the stuff windows are made of. Upon adding heat energy endothermic
or exothermic reactions will be observed.
It is kind of like a phase change but softer. To remove a structure like
crystals or sodium glass fine order a flattening
of the energy vs temperature curve will be observed. The ultimate of course
is a phase change where temperature
remains constant over a range of added energy. Disordering the structure
absorbs energy therefore is endothermic. We call
this the glass transition temperature. If there is little or no structure
the specific heat will be more constant.

It is very important in wood finishing to have the glass transition
temperature of the finish much lower that room
temperature. Then the finish can be seemingly hard but will
cold flow without cracking to accomodate wood movement.


I should mention that even liquids have some structure. They sure do not
behave like ideal gasses!

This suggests to me that if anything (window) glass at room
temperature might tend to crystallize. The crystalization rate must be very
slow though.... I have seen ancient pieces of obsidian (natural volcanic
glass) that did not crystallize after millions of years!

At some higher temperature no doubt window glass is a thick
liquid.

It is a little off topic but thats fine.... Learning about the materials we
use is a good thing like I said. I like to learn
about them too so I study geology, metalurgy,and minerology for fun.

I am going to stick to other materials for cnc tables though!

Leslie Watts
L M Watts Furniture
Tiger, Georgia USA
http://www.rabun.net/~leswatts/wattsfurniturewp.html