The Mysteries of Dichroic Glass
By Brian Kerkvliet © 9/20/96
In the last several years there has been a growing interest in the use of dichroic glass. There are many reasons for the attraction to the spanky glitter effect that it produces. The most common being: "It sells my work", "I love the way it looks", "Its cool", " its non toxic", to name a few. In many ways I feel that the material is over used to create "pretty baubles" that will sell! The material it's self is relatively easy to work with if you follow several basic principals, so glass workers, especially beginners, are enthusiastic to glob it on their work to make it desirable. Dichroic is in the nineties, what silver chloride was for hot glass blowers in the sixties, an easy way to add color and intrigue to the piece of glass. I feel there is no substitution for good technique, whether you use dichroic or not, you should practice good technique. If dichroic is used in your work with this in mind, it can be used in many ways to help you express your self by adding a magical element to your work. In other words use dichroic in your work, don't let dichroic be the work. That's too easy!
With this said, let me explain how the coatings are applied to the glass and what they are made from, this will help you understand how this 90's material can be used most effectively to add some magical glitz to your pieces.
THE COATING PROCESS
Dichroic glass is a high-tech spin-off of the space industry. "Dichroic" is defined as the property of having more than one color, especially when viewed from different angles or from transmitted to reflected light. Dichroic coated glass is produced by a process called "thin film physics" and is generally referred to as a color separator. It's normally used as an interference filter in scientific measuring or correcting applications. To get this effect, thin layers of metallic oxides, such as titanium, silicon, and magnesium are deposited upon the surface of the glass in a high temperature, vacuum furnace.
The glass to be coated is carefully cleaned, and fastened to a planetary arm in the top of the furnace chamber. The oxides are placed in a crucible on the bottom of the chamber. Air inside of the chamber is removed with a high vacuum-producing cyro-pump, and the chamber is heated to 300 °F. The metallic oxides are vaporized by an electron beam, and the rotating glass target is evenly coated with many thin layers. The resulting color is determined by the individual oxide compositions and the coatings sequence, totaling about 700 angstroms thick, (3 to 5 millionths of an inch). All of this is tightly controlled by a computer. Since the total thickness is so minute, the filter has very little mechanical integrity of its own and must be supported on a mechanically stable substrate. Glass is the ideal candidate for this substrate. It is transparent, has adequate rigidity, is stable, withstands relatively high temperatures, and is not affected by moisture, solvents or most acids.
Dichroic coatings transmit certain wavelengths of light, while reflecting others, thus creating an interference-effect similar to the iridescence observed in natures fire opal, dragonfly wings and hummingbird feathers. The transmitted color is different than the reflected color, and when the light rays transmit straight through the glass they are less effected by refraction than when passing at an off axis angle, which makes the light travel a greater distance through the glass. It is this distance that causes a color shift. When dichroic glass is viewed at even slightly different angles, you will see a variety of different colors as you rotate it. When you rotate the filter from viewing directly through it the intermediate colors will shift down the rainbow. The colors of a true, natural rainbow produced in our atmosphere are usually considered to be in the sequence of vlolet-blue green-yellow-orange-red. In the typical simple single stack dichroic glass design used by most manufacturers, the reflective rainbow is similar to the natural rainbow. The transmitted rainbow is, however, in the sequence of yellow-magenta-blue-cyan. Since there is no green or red available in the transmitted rainbow a second more complex two stack design is often used that provides a transmitted rainbow similar to a natural rainbow. This allows the production of a sheet of dichroic glass that transmits green or red.
Fusing and flameworking artists can use these colors as long as the implications of heating the dichroic coating are taken into account. When bringing the coating to melting temperatures the coating will craze and the color will move toward the yellow/purple end of the rainbow. This is a permanent color shift and for typical working operations the color will shift only one color towards Yellow/Purple. One needs to be aware that the color shift is dependent on temperature, exposure time and number of reheats necessary to finish the piece. So when you are using coated glass and you want the finished piece to have a reflective blue color, you would want to use a green reflective coating so that when it shifts you will end up with the desired color
The application of sheet dichroic
There are several different ways to apply sheet dichroic coated glass to your piece with a variety of different results. I will mainly focus on the flameworking applications of dichroic glass, however many of the same principal apply if you are to fuse with it.
You can get dichroic coated on a variety of sheet glass. Moretti c.o.e.104, Bullseye c.o.e. 90, Spectrum c.o.e. 96, Uroboros c.o.e., 96 & 90, and borosilicate c.o.e.32. These different base glasses are only the substrata used to convey the coating to your piece of work. So you must use the same coefficient sheet as the work that you are going to make. Most all of these glasses are normally available in clear or black. With the exception of the borosilicate and the Spectrum which are only available in clear. Some suppliers may carry a few coated rods as well, but I feel that it is easier to work with the sheet to get better results. Usually when you get dichro sheet from your distributor it is in a 16" by 16" square. (Unless you can only afford the 6" x 6" piece.) This is the main sheet that you work from. I normally cut several strips off of the sheet in a variety of widths. The strips that I cut from the soft glass are anywhere from 1/8" to 1" wide. If you get too much wider then that, you run the risk of thermo-shocking the piece when you put it into the flame. I usually fire Moretti or Bullseye dichroic strips in a kiln to 1350°-1400°F, just hot enough so that the edges soften a little bit and the coating crazes. This seams to help adhere the coating to the glass so it's less likely to burn off when you work with it. For the coated borosilicate I don't fire it in the kiln because it doesn't seem to need that extra baking to work nicely, besides you would have to fire it very high to get the same effect. You can get away with cutting wider strips if you want to wrap a whole tube with it. It is less likely to thermo-shock when you put it into the flame.
Before you apply the sheet it is important to know how to tell which side of the glass has the coating on it. You can tell by looking for the most reflective side of the glass, or if the edge is chipped you can see if the metal has flaked off the coated side, and last but not least you can tell as you put the piece into the flame, the coated side will start to craze when it gets hot. Eventually it will become easier to tell just by looking at it.
The coating is very susceptible to burning if exposed to certain atmospheres of the flame, in particular a reducing atmosphere. (Too close to the torch or too much gas.) This is one of the reasons that you want to be sure that the coated side of the glass is very clean before you start applying it to your piece. If there is any residual oils or dirt on the surface it will create a reducing atmosphere as it burns off, and this will negatively effect the coating. Over heating the glass can also fry the coating off of the piece, so for best results use a flame that is not too hot and isn't reducing. The length of time that you work the glass is also a factor. It seams that if you work the glass for a long time you can end up shifting down the rainbow and then on to oblivion... a gray\white scuzz. So keep your working time as short as you can.
Because the dichroic coating is only applied to one side of the glass you can get two different results depending if you apply it with the coating side up or the coating side down. The coating creates a surface tension on the glass, which means that it won't flow as smoothly compared to the base glass that it is applied onto. You can place the dichro onto the piece with ether the coated side up, or the coated side down. If it is put on coated side up, you will have to be a little more careful about burning it off. That means you will have to work quickly, in a non-reducing flame, that isn't too hot. With the coating up you will get a mat surface with a grainy texture from the metallic coating breaking up into little flecks. This has a nice effect, but I find it difficult to blow the piece out as far as I can when the coating faces down. On solid pieces it is a little easier to have the coating side up.
When you place the dichro on with the coated side down, the surface remains nice and glassy and there is less of a problem with burn out, since the coating is sealed in. Because this is actually a metallic coating it doesn't stick to glass the way that glass sticks to its self. It is somewhat challenging to get the glass to stick nicely without over heating the coating. To do this you must first heat the piece that you are going to apply the strip onto , then heat the dichro mostly on the non-coated side, but you have to give the coated side some heat so that it will stick to the piece. After you have successfully applied the dichro to your piece face down, often times the edges want to curl up or pull back from the piece a bit. This can be fixed by heating the afflicted area, and using a knife or spatula type tool, push the edges down and out onto the piece. If the edges aren't sealed onto the piece there is a good chance that a portion of it will curl up and the coating could burn off leaving a gray scuzz. We don't want that now do we? The other thing that I do sometimes is to seal the edges by striping a stringer of clear or colored glass along the edge, wide enough so that it connects both the dichro and the body of the piece. This technique can add an interesting effect of highlighting the coated area in your piece. All of these principals hold true whether you are applying them to solid or blown pieces.
Working dichroic on blown forms.
When you apply sheet dichroic strips to a tube that you plan on blowing out there are several things that you need to keep in mind. The most important is that you are applying a metal to the surface which doesn't move the same as the rest of the glass when it is stretched. In fact the dichro will not stretch at all, instead it will break the coating up into small little platelets that will float away from each other. So don't put the dichro where you plan on blowing out the tube a lot, because the coated effect will be all but lost if its stretched too far. The metallic coated glass must be applied evenly to minimize the effect it will have on the way the piece blows out. If you put several small pieces of dichro on the outside of a tube you are in effect doubling the wall thickness in that area, not to mention the added stiffness created by the coating it self. These pieces that you have applied to your tube will tend to act like islands in a soft ocean of molten glass when you go to melt them in. This effect can be lessened to some degree if you add other color or decoration to the negative space between the pieces of dichro. This will tend to help even out the wall thickness which makes everything melt in better.
Working dichroic on solid forms.
Generally working with dichro on solid glass is easier than when you use it on blown pieces. You don't have to worry so much about the coating getting over stretched, unless you plan to pull a rod that has dichro on it , like a twisty cane. The surface tension problem isn't so much of an issue ether, because your piece tends to heat up at the same rate as the coated glass and you have a better opportunity to work it into the surface. Many times I like to lay on a strip of dichro and stretch it as I lay it on. Then after it is on I use the small spatula again to stretch the coated area out from the edges as if you're stretching a hide. Work a little bit on one side by repeatedly heating and manipulating that area, then move on to another area until the whole area is stretched evenly. Then I peal away any of the edges that look like they have a bit of scuzz on them and continue to work the area flush with the surface. When the coating is face down the main thing that you need to focus on is not trapping air bubbles between the dichro and the piece as you apply it, and cleaning up the edges after it is applied. Other then that you will find that by using this method, rarely will you have problems with the coating burning off.
After you have successfully applied and cleaned up the edges of a dichroic section you can start applying other detail that will be part of your design. The sky is the limit of how you can use dichroic glass. You can place it on top of a dark color like cobalt for strong results, or you can cover the coating with a soft transparent color to subdue the sometimes over powerful glare that it can have. Many times I apply it as a background and add the main focus of the design on top of it so that it creates a three dimensional effect. Other times I use it in very small wisps. Strips that I have stretched out so that when the little platelets drift apart they look like fairy dust in the background.
With a little practice and some imagination you can use this wild material to heighten your creative vision and explore where no one has gone before! I hope that this article has helped you understand the potential uses and misuses of dichroic so that your work can be all that it can. If you have any questions or slides of your work that uses dichroic in a new and interesting way, send them to me c/o Glass Art magazine. and I will take them into consideration for inclusion in future articles on this topic.
Until then , Flame on!
A list of coaters and suppliers
COATINGS by SANDBERG
GERALD and NONA SANDBERG
900via Rodeo Placentia, Ca. 92870
Fax: 714-572-8659 Email: email@example.com
Pyrex and clear 90 coe dichroic sample sets, sheets and custom coating
800 Ploma Dr., Ste. 100
Round Rock TX 78664
ph: 512 246-1122 fax: 512 246-1133
96 and 90 COE compatible sheet and sample sets.
D&L Stained Glass
Northwest Art Glass
C & R Lou
Thin Layer Coatings
15635 SE 114th suite 106
Clackamas Or 97015
Dichroic coating on all types of glass
Frantz Bead Company
E. 1222 Sunset Hill Road
Shelton WA 98584
ph: 360 426-6712 Fax: 360 427-5866
Moretti and Pyrex dichroic sheet and rod
125 W. 1057
Gardina Ca. 90248
Custom dichroic coatings on all types of glass