Polarisation in cast pieces.

[Lauri Levanto]

I do not have proper polarisationeqipment yet.
Testing with a pair of sunglasses I have got an
impression that the refraction of light in a cast piece with strongly curved surfaces causes polarisation
patterns, too.

Am I right or have I failed in annealing?

-lauri

[jerry flanary]

Are you
a) shining light through the glass?
b) holding a polarizing film between the glass and your sunglasses?
c) are you spinning the film until it is 90 degrees off from your glasses? If you are you should see a shadowy cross.

Any rainbow types of color are stress (compare against a clear plastic tape dispenser or a plastic coke bottle- squeeze this object and you will see the stress)

What the stress actually means or if it is an acceptable amount of stress is tricky as it varies from polariscope to polariscope. So you'll have to make your own notes...

The shape of the glass should not affect your viewing, but there are always exceptions. I just cant think of one.

[charlie holden]

It can be really tricky when a piece has lots of curves or relief. Transitional areas pick up light from some distance and shine more brightly than the surrounding glass. I've never heard of normal glass actually polarizing light, though it can twist already polarized light.

ch

[Lauri Levanto]

> Charlie
> It can be really tricky when a piece has lots >of curves or relief. Transitional areas pick up light from >some distance and shine more
> brightly than the surrounding glass. I've >never heard of normal glass actually polarizing light, >though it can twist already polarized
< light.

> ch

The twist is what a polariscope detects.
Physics is not my strongest science, but I believe
that as polarized light reflects differently,
the thru going light has more the other polarity.

When sunlight is reflected from a horizintal sheet of glass
(or water surface),
the reflection is polarized and filtered off by polaroid sunglasses.
The rest of light, going thru the glass or water
must have a residual polar distribution.

In a curved 3D glass object the same happens in
different directions, not only horizontally.
As the transmitted light varies in intensity,
it looks like variation in polarisation.

-lauri

[jerry flanary]

Sorry
I made a little mistake in my description of the process. It is light, polarizing film, glass, your glasses. You have to polarize the light, run it through the glass, then look through your lenses. The contours of the glass shouldn't be a big deal. You have to run the light through two filters though. Is this what you are doing?

[Bob]

From the dark recesses of my memories of second year mineralogy back in 1970 I seem to remember (through the haze of the 70's) how polarization works.

One of the theories about light is that it propogates as vibrating waves. A polarizing filter removes all directions of vibration except for one direction ... say north-south. So the light that passes through the first filter vibrates only in a north-south direction. If that light passes through a second filter where the preferred vibration direction is east-west then there is no transmission of light through the second filter. There are only n-s oriented waves... no e-w waves. The field of view is dark.

Incompatibility between coefficient of expansion in glasses causes a residual stress/strain at the boundary between the glass types. If glass has no residual stress then light waves will pass through the glass without any changes in vibration direction. However, if there is residual stress within glass, then as light passes through the stressed area the wave will bend. So if light only vibrating n-s enters a stressed area it might bend slightly from a pure n-s orientation. When that light leaves the glass and passes through the second filter (oriented e-w) then there will be a small component of n-s light. This will result in a light halo. The greater the amount of stress, the greater the amount of bending, and therefore the brighter the region of transmitted light.

I have not done much testing of glass with polarizing filters. The little that I have done suggested that the surface of the glass had to be flat . Otherwise (and I am guessing here) the light would refract as it emerged from the uneven surface. This refraction might cause an apparent bending of light.... which in turn resulted in light regions on the polarized filter. This last part is pure conjucture. I hope that some experienced casters who have tested their pieces for stress can provide a more reliable answer than the refraction guess. The first part, on the theory, is (despite the haze) pretty accurate.

Cheers,

Bob

[Lauri Levanto]

Jerry, up to now I have to use two pairs of
polaroid glasses, the other one behind the glass
turned 90 deg to the darkest position.

Bob, the intensity differencies thru a sculpted
form seem to disturb the halo enough.

-lauri

[charlie holden]

The twist is what a polariscope detects.
Physics is not my strongest science, but I believe
that as polarized light reflects differently,
the thru going light has more the other polarity.

When sunlight is reflected from a horizintal sheet of glass
(or water surface),
the reflection is polarized and filtered off by polaroid sunglasses.
The rest of light, going thru the glass or water
must have a residual polar distribution.

In a curved 3D glass object the same happens in
different directions, not only horizontally.
As the transmitted light varies in intensity,
it looks like variation in polarisation.

-lauri

Lauri,

The thing that I would quibble with here is that you say a reflection is polarized. It is actually just the opposite. The light off a reflective surface is scattered -- the waves are all oriented in different directions. Scattered light also comes at you from a lot of different directions since the surface area of the reflection is large. The polarized sunglasses then cut out all the waves other than those lined up in the same direction as the grid lines of the lenses. The light isn't polarized until it goes through a filter.

I think you are right though, that a curved piece of glass can pick up light that isn't polarized, and seem to glow as if it is stressed. I spend lots of time looking at cast pieces trying to decide if I'm looking at stress or just a reflection.

ch

[Lauri Levanto]

Charlie,

To be precise, the reflection is not polarized, but
the light that has polarity perpendicular to the
reflecting surface is reflected more than the light
with parallel polarization.

The light is not polarized, but the distribution
of polarized light is no longer even. That is why
Polaroid sunglasses work so well at sea.

Just test with a polarisation filter. Look at some
slant reflection and turn the filter 90 deg's.

To be precise, nor does a filter polarize light.
It only absorbs 'wrongly' polarized light,
letting only selected range thru.

-lauri

[jerry flanary]

Charlie,
why not look at your glass in a room with no other source of illumination. Then you could sit in a dark little room for hours and at least have a legitimate excuse when you came out. (Craftweb has better emoticons.)
j.

[charlie holden]

Charlie,

To be precise, the reflection is not polarized, but
the light that has polarity perpendicular to the
reflecting surface is reflected more than the light
with parallel polarization.

The light is not polarized, but the distribution
of polarized light is no longer even. That is why
Polaroid sunglasses work so well at sea.

Just test with a polarisation filter. Look at some
slant reflection and turn the filter 90 deg's.

To be precise, nor does a filter polarize light.
It only absorbs 'wrongly' polarized light,
letting only selected range thru.

-lauri

I did not know that differently polarized light reflected differently, but it makes sense. Though, to be precise, I think that the polarity has to be defined as parallel or perpendicular to the plane described be the angle of the reflection rather than the plane of the reflecting surface.

Perhaps one can say that light of a certain polarity is reflected uniformly while light of other polarities is scattered more widely? Here's a reference for those that like to be blinded by science:

http://en.wikipedia.org/wiki/Polarization

ch

[charlie holden]

Charlie,
why not look at your glass in a room with no other source of illumination. Then you could sit in a dark little room for hours and at least have a legitimate excuse when you came out. (Craftweb has better emoticons.)
j.

Jerry,

The problem is, my source of illumination is so big that I haven't been able to find a filter that can cover it completely. I like my light, a lot. No dark rooms for me.

ch

[Bert Weiss]

I'm not sure exactly why it's better, but I have a glass blower friend who has a commercially made polarizer. It is a can with a light source and one polarized filter. The second filter is on an arm that sticks out far enough to place a glass object between the filters. On his unit the stress is much clearer to see than when I use my plastic polarized filter sheets that I got from Edmund Scientific. My sheets are not perfectly flat and parrallel which is likely the difference.

[jerry flanary]

Hey
I got some sheets 8 1/2" x 11" on ebay. cheap.