Showing posts with label Dichroic. Show all posts
Showing posts with label Dichroic. Show all posts

Wednesday 1 August 2018

Dichroic coatings


Description

“Dichroic glass is a multi-layer coating placed on glass by using a … vacuum deposition process. Quartz crystal and metal oxides [such as titanium, chromium, aluminium, zirconium, or magnesium] are vaporized with an electron beam gun in …[a] vacuum chamber and the vapor then floats upward and … condenses on the surface of the glass in the form of a crystal structure….  [As] many as 30 layers of these materials [are applied] yet the thickness of the total coating is approximately 35 millionths of an inch.”  http://www.cbs-dichroic.com/faq.asp

“This coating that we commonly call dichroic glass today, is actually an “interference filter” permanently adhered to the surface of a piece of glass. The technology used to manufacture the optical interference filter has been in existence for many years. It is known as vacuum thin film deposition“  Howard Sandberg.  http://www.cbs-dichroic.com/fyi.asp

“The total light that hits the dichroic layer equals the wavelengths reflected plus the wavelengths passing through the dichroic layer.  A plate of dichroic glass can be fused with other glass in multiple firings. Due to variations in the firing process, individual results can never be exactly predicted, so each piece of fused dichroic glass is unique.”  Wikipedia



Care in Use

Dichoric glass can be used in stained glass as well as kilnforming.  There are some precautions to be observed when handling dichoric coated glass.

Determining Coated Side
The coating is a thin film that can be damaged easily. So, the first thing is to determine which is the coated side when the film is on a clear base.  One way is to look at the glass at a very acute angle.  If you see the colour above the clear, the coating is on the top.  If the clear is above the film, the coating is down.  Another way is to put a sharp point in contact with the glass.  View at a sharp angle.  If the point appears to touch the surface, the coating is up.  If there appears to be a small space between the point and the surface, the coating is on the bottom.  It is normal to check both sides to confirm the first impression.  Of course, if the dichoric is on black, the coated side is obvious. A more complete description of the method is describe here.

Scratches
The dichoric film is strong, but very thin.  This means that anything that could scratch the glass will also scratch the coating.  Avoid the use of abrasives when cleaning the coating.  This means that steel wool and harsh abrasive cleaners should not be used.

Scoring and Breaking
As the film is very thin, it is best to cut on the non-coated side.  This avoids any chipping as you score the glass, and provides a clean break.

Grinding
Also, when grinding the edge, you should use a fine grit to avoid chipping off the dichoric.


Fusing Notes
The dichroic coating is a strong thin film that does not expand and contract to the same extent as the glass being fused.  

Avoid movement
When there is a lot of movement in the glass, the coating can split. If the dichoric is on a clear base, you can fire it facing down to reduce the fracture of the film. You can also fire it with clear glass above to reduce the stretching and tearing of the dichoric film.

Over firing
Firing too hot causes additional movement in the glass, so you can think about reducing the temperature to avoid that over firing, which causes lots of movement of the glass.  You should also think about the volume.  If there is more than 6mm of glass, it will begin to spread to reach that thickness.  The spread causes a stretching stress in the dichoric film that can cause it to break apart.

Orientation
You should not fire with the dichoric faces together.  The films do not fuse together, so the glass bases and tops will act as single layers and pull in, creating multiple fractures in the coatings.



Frit
In addition to dichoric coated sheet glass, there is also dichoric coated frit from CBS.  They have designed a proprietary process that allows the frit to be coated on approximately 80% of the surface area of the frit. Due to this high ratio of coating versus glass, the dichroic frit responds very differently under heating/hot working conditions.  Based on: http://www.cbs-dichroic.com/faq.asp



Wednesday 23 December 2015

Score coated glass on the back - Kiln Forming Myths 16

Cut iridised glass on the back


The idea seems to be to get a more even score and avoid chipping of the iridised surface.

First, the iridised surface is almost microscopic in thickness.  It is put onto the surface as a mist of metallic oxides as it begins its run through the annealing lehr.  This thickness will not affect your scoring.

The back is usually rougher side of the glass and so will be more difficult to get a smooth, even score than the front iridised surface.

Chipping of the iridised surface is caused by too much pressure during the scoring.  Reduce your pressure and review your scoring practice.


Scoring the iridised surface with appropriate pressure will produce a clean break without chipping the surface.  These comments apply to dichroic and flashed glass too.