Binder in Glass: how to avoid color issues in "glass clay" Part I
Updated: Feb 7, 2020
Before I get started on complete voyage into this subject, let me show you why glass clay is awesome. You can defy gravity, get sculptural and push the envelop in glass with original art that is born completely from your own imagination. You can also use precision silicon and plaster molds (your own or made by others). It's an incredibly flexible skill to develop. Glass clay can do what many other glass techniques do, only with more control for the artist. And it can do what other techniques can not do.
Can your vitro graph do this? Not bad for just playing around a bit:
Are you embarrassed by "cold fusion" (aka using glue)? What if I told you that you don't have to do that any more? I made this flower out of scrap material and fired it with out support. It is a great proof of concept. It wouldn't take much to add a stem, leaves, make a bouquet cinched with glass ribbon, or run cooler for less slumping or support the petals for what ever shape I want. It is entirely fused and has wonderful variance of textures. What ever I wish I can make with out the embarrassment of gluing.
Can your frit casting or press mold do this?
Under cuts and over hangs? No problem what so ever.
Want to use gravity, plasticity, surface tension to your advantage with out the limitations of a mold? This birds nest on the right was fired multiple times with out a mold.
This mermaid sits on a coral nest on a half shell like Venus herself. She and her coral have undergone multiple firings with out loss of shape, sagging or loss of detail. Everything is 100% fused. There is no "cold fusion".
The versatility and flexibility of glass clay in glass can not be overstated. It is a breakthrough paradigm, with the potential to take your work beyond casting, lost wax, mold making, vitro graph, stringer weaving, traditional Patte de Verre, etc. But like all things it has a learning curve and some challenges.
This summer I made some really fun Patte de Verre creations. Unfortunately after firing I was spectacularly disappointed with the colors of my glass. Below is a photo of a bird's nest that has been through 3 firings with bird, leaves and birch tree branches that have been fired twice. Note the ugly gray pervasive in all the colors except black and turquoise. I had been told that I just needed more firings since either I hadn't burned off all the binder or my colors had failed to strike. That seemed unlikely considering the length of total firing, temperatures and color issues with non-strikers. So I became briefly obsessed with binders and colors. I use to work in Research and Development as a Chemical Engineer. Love of experimentation is a part of me. So much so, I sometimes get side tracked running experiments on my glass. Good thing I have a big kiln since components for art often share half the space with tests! I can tell you with confidence the gray problem is neither a failure to strike nor a failure to burn off binder "long enough". After extensive color testing I can say that if a color is gray after the first firing it will be problematic and risky. A second firing (or fifth) may help somewhat but it also may not help at all. Always test colors. Work with colors that you know are going to look good after firing just once and you will be happy.
Now for the promised voyage:
About Binders: Many glass artists are exposed to food making and cake making/ decorating, so many ideas from that world migrate back into glass. Glass powder is similar to sugar, but not as helpful as sugar. Sugar partially solubilizes but glass remains stubbornly its self, like sand in frosting. Depending on what you are doing, you might be using a shit load of binder in order to get a frosting like consistency with powdered glass. So it is important to understand a pinch of binder chemistry.
For example, I have never had a color problem or a residue problem when using Bullseye Glastac. It's great stuff as far as I am concerned. According to the MSDS it has: Hydroxypropl Methylcellulose, Water, Sodium Chloride, Proprietary polyglycol, Carboxylic Acid, Aldehyde & Trade Secret. What catches my eye - long chain molecules, reactive atoms like Na and Chlorine and potential for other un-named minerals - particularly from the methyl cellulose molecule. When I use bullseye glue I use just a drop or two, ok maybe a line of it at most. As sold, it's a highly dilute solution. I know it's highly dilute because I know what methyl cellulose feels like when you hydrate the powder (which you can purchase on Amazon for making food). I also know it's made by Bullseye so it has been tested for burning away cleanly and it is compatible. Many users over many years with consistent, replicable results makes this scientist very happy and confident. I am not putting a tablespoon of the stuff into a small volume of glass powder in order to make a dough or batter either. I am not dehydrating it into a thicker gel before use. I also know that the auto ignition temperature of methyl cellulose is about 700F. As stated, colors are true and I've never had a problem.
So first rule of binders - less is better, at least in terms of glass color after firing.
I have also done a more traditional Patte de Verre where fine glass is used and just a little binder to hold the fine together against the support of a plaster mold. In the class we used every conceivable color fearlessly. We mixed custom colors. There was never an issue with color X binder. Naturally when you fire Patte de Verre you are making a porous body so there is an opacity difference from firing hot for a complete melt. You do have to think about strikers and glass on glass reactions. However; there is no color graying issue. Hence clear glass is used for white some times. Using only a little binder is best for excellent post firing colors.
So what is happening when you use a large amount of binder? Why so much gray? What are the risks and how can one avoid them?
If you've ever made slides in biology then you know where I'm headed. Below is what methyl cellulose looks like with a hydrated blob of just binder is plopped onto some clear glass slides. The left side of each slide has a single layer of glass with a blob on top. The blob is exposed to air for the best combustion conditions. The right hand side has a blob sandwich. The top plate of glass simulates mass transport limitations with air for the interior of a glass clay creation. The slide on the left had one firing. The firing was conservative in terms of time spent above autoignition temp. The slide on the right had two firings. As you can see a large molecule needs time @ temp to burn out but eventually there is residual that stays put even after additional firings (third, fourth). Long hold times for burn off, high top temperatures and slow ramp rates won't necessarily save you either, but they are a best practice.
Bottom line: more binder means more risk of noticeable mineral deposits.
The combustion reaction is:
Heat + Carbons, Oxygens and Hydrogens make Water (gas) and Carbon Dioxide (gas)
Heat + Cs, O's H's --> CO2's g + H2O's g
If C's and O's and H's are all you have in your binder, you can achieve complete burn out from the glass. Thats the good news. The bad news is any minerals (Mg, Ca, K, Na etc) will not gas out in appreciable quantities at firing temperatures, but will form deposit that may impact the optical properties of the glass in some way.
First - most binders are made of large molecules. Xantham Gum has a molecular weight around 2000 (!) for example. What that means practically is - larger molecules and poly cyclic molecules need more heat and time to decompose (burn out) than smaller molecules and molecules with short, unsaturated chains.
Second - going back to the shit load, many binder + glass recipes call for a lot of binder to get the glass to behave in gravity defying ways. Consider a recipe calling for 1/3 teaspoon binder powder per tablespoon of glass powder - you are then expecting 10% by volume of your creation to burn out. This means shrinkage & movement from shrinkage. It also can mean a lot more Na or Ca or Mg or what ever atoms the binder molecule contains that won't burn out. It's also likely that with more binder in a powdered glass structure, there will be some mass transport limitations that will require more time at temperature to assure you react (burn) out as much binder as possible. I have noticed that edges and elevated glass can have better color and I have cut fired glass to see if the color is better on the surface than the interior. Also it is very important to mix the glass with the binder very well. Don't have lumpy gravy.
Third - mineral content is important. I would recommend minimizing the content as much as possible because the minerals don't burn out, they deposit. It's possible they form salts, it's possible they form oxides, or a combination of both. Bottom line - there is residue that impacts how your glass looks. Unfortunately with some binders, that means gray or muddy colors that won't improve much if at all in subsequent firings. As a minimum go with no added preservatives.
Second rule of binders: Always test your colors and keep track of what color and what binder you used. Note the graying of the mint green and transparent gold purple and the light mottling on the Aventurine green.
I like to set up a slide on clear glass with just a small pile of pure glass in the colors I am firing so I can test strike and color trueness. I can then compare the binder's effect on color vs pure glass under the same firing conditions. I also will sometimes use a white sheet glass as the background (Choose non reactive or buffer with a thin film of clear powder). I can say that I have never had an issue with striking using my firing schedules.
A different binder first and second firings below and color tests. This binder gave much better colors after a first firing, including opaque white (which still looks too gray for my taste).
When working with large amounts of binders, you need to consider mass transport limitations. Imagine going to a party with many people. You have to wait to get through the door as some people are going in and others are going out. The door makes a bottle neck. This is what is happening inside the powder as binder burns out and leaves and oxygen from outside comes in to keep the burning out reaction going. You will need time and temperature in your firing schedule to support the burning out process. If you don't have enough time and temperature, you will not only have minerals left in your glass, but also incompletely combusted carbons and hydrogens which make up most of the molecule of binder. That holding time and temperature will also need to comport with the dynamic properties of heated glass and what you ultimately want to achieve for your artwork.
Third rule of binders: Just as you need to hit strike targets and fuse together targets, you also need to hit burn out targets. Make sure you have enough burn out time in your firing schedule. Don't leave anything on the table.
People try different binders to impart different properties onto the glass mixture. This is a great idea. It is also a way to expand the color pallet, as long as you have an idea of how to fire and what to expect after firing. Currently I am using 4 different binders, depending on what I want to achieve. They are tools as different as the paint brush and the dental pick.
Fourth rule of Binders: Have multiple binder tools for a full array of colors.
I hope this was helpful. I will consolidate a table of binders vs color results eventually and keep adding as I go.
Speculation - I did discuss this with a friend who has a PHD in materials science. We speculated that different reactions for different deposits may well happen depending on hold temperature range and some deposits might play nicer. I did try going quickly to a hot top temperature hold then slowly cooling and holding around 1225 for good striking but did not see a worth while color improvement. I did notice that at contour fuse temperatures I had an improvement in the transparent gold purple, however sunset coral retained gray mottling and neo lavender opaque remained gray. That said, if I want a particular effect and retention of detail, hotter temps such as fire polish through full fuse won't be an option. On the other hand some interesting effects can be attained at these higher temperatures with "glass clay" and the color palette for these effects looks like it will be expanded vs colors that will play well with cooler firings. Finally for what it's worth, some colors do improve upon 2 or more firings, such as light pink striker and burnt scarlet but holding at a longer burn out temp for one long firing with the same amount of "burn out time" did not produce the same nice color results. There may be some advantage passing through the transition temperature range more times (valence state changes in the glass range - maybe some minerals make it into the glass atom?) I did also find that clear cleaned up with 5 firings to something attractive. I did also read that from the strain point upward through the transition range the bonds in the glass atoms are breaking and changing valience states so it may be that perturbing the glass through the plasticity process is in some way beneficial. Perhaps some atoms can be absorbed into the glass molecule? But this is again pure WAG. Again my advice is to keep track of firing schedule vs color results so that you will have pleasant and predictable results vs unpleasant surprises in your finished art.