Brad Shute's Wicked Good

Question and Answer Page
* Making Glass *

Questions and answers about glassmaking.

* Updated November 10, 1998 *

Start Page | Navigation Page | Glass Discussion Board

Bottom of This Page

Following are questions I have been asked about glass, along with my answers to them. If you have glass questions, feel free to e-mail them to me. I will add them to this page as time and content warrant.

Q. Do you ever get burned?

A. No.

Q. Do you ever get cut?

A. No

Q. Did you answer those first two questions honestly?

A. No. :::Grin::: Actually, cuts and burns are an occupational hazard, but can generally be avoided if one pays attention to what one is doing. I very seldom get burned, and then usually only because I was distracted by someone else in the studio. With just a small amount of foresight and care, 99% of burns can be avoided easily. I learned a simple fact early on. Burns hurt! So I avoid them...
Cuts are a slightly different story. Usually, if I get cut, it happens when something breaks unexpectedly. I try not to do anything stupid to cause things to break, but sometimes it happens. So, while they are infrequent, cuts are harder to avoid. They are usually worse than burns too. I have required a few stitches in the past, but it has been quite a while since the last time. Experience pays off. Beginners can count on getting cut AND burned...

Q. What is glass made of?

A. The glass I make is made from a mixture of sand, soda ash, and powdered limestone, with a small amount of other stuff throw in to adjust the qualities (ease of melting, clarity, color, etc.). The mixture (called "batch") is shoveled into the 2300 degree furnace, a bit at a time, over a period of about six to eight hours and then cooks overnight at 2400 degrees F. Everything melts together and liquifies, and then you have glass. After it is melted the furnace temperature is turned down to about 2100 degrees, and the glass is ready to use. At 2100 degrees, it is the consistancy of honey. It gets more viscous as it gets cooler and less viscous as it gets hotter.

There are many different glass compositions, each with a particular set of working/melting characteristics. Some melt and get very watery at much lower temperatures than mine, and some specialty glasses will not soften at temperatures that will melt steel. It all depends on the composition.

Q. How does the glass acquire color and are there special pigments, different than - lets say - pigment for paint or ink?

A. Different chemicals and metallic oxides are used to color glass. Some are the same as those used to color paint and ink. Colorants for glass run from precious metals such as gold (gives a beautiful - but expensive - ruby color) to chemicals such as arsenic and sulfur (used in the past for white, and used to make reds, yellows and oranges, respectively). The composition of the base glass as well as the melting conditions (temperature, oxidizing or reducing atmosphere in the furnace, heat treatment, etc.) make a big difference in how various colors work. Because of this some colors are very difficult to make consistently. Others are easy. An example of an easy one is cobalt blue - you can do everything wrong and you will still get blue glass. Gold ruby is a difficult one, as are many of the reds and yellows.

Q. I'm still perplexed about color/pigments. For one thing when and how does it become part of the glass - when the glass is being melted in the furnace or afterwards somehow?

A. The chemicals that color the glass are part of the batch (the sand/chemical mix) that is melted to make the glass. Because different colorants react with the base batch (the batch without colorants) in different ways, the base batch ingredients are usually adjusted slightly, depending on the color being made.

Q. You said that blue was an easy color to come up with and I'd always thought (perhaps erroneously) that blue oxidized faster than other pigments. So does easy mean that it's always the same exact blue or always in the cobalt blue family?

A. I am not sure what you are referring to as far as blue oxidizing faster than other pigments (paints perhaps?) but in glass it is almost impossible to burn out a cobalt blue. Many, if not most, glass colorants will lose their color if subjected to enough heat during either melting or later reworking the glass. This is not a problem with cobalt oxide. Cobalt blue glass holds its color no matter what you do to it, which makes it an easy color to make. The slightest bit of cobalt added to an otherwise clear batch will give a pale blue glass when melted. To get a darker blue, one simply adds more cobalt. It doesn't take much cobalt oxide to darken it, either.

To make a very dark cobalt blue glass, I mix less than 1 oz. of cobalt oxide with 50 lbs. of clear batch. That's about the saturation point for the batch that I use. After that, adding more cobalt doesn't change the color any more, so adding more is just wasting it. Add less and it gets lighter, but after a certain point adding more won't make it darker.

Blue glass can also be made with copper oxide, which is much more sensitive to the furnace atmosphere than cobalt.

Q. Can you match colors exactly from day to day?

A. Some colors don't change in the furnace, but many do. Once again, cobalt is good in this respect, whereas reds and yellows can be difficult. A gas fired furnace is more likely to change a color because of the flame and its effect on the furnace atmosphere.

Q. Where do you get your colored glass?

A. Like many other glassblowers, most of the colored glass I use is imported from Germany or Italy, with a small amount coming from the U.S. and other places. I have made some myself in the past, and will do so again when I finish building my new color furnace. Once the color furnace is running I will also melt colored cullett (recycled glass) from U.S. manufacturers, so I have a wider color pallette.

Q. Why do you buy foreign colored glass instead of glass made in this country? Isn't it expensive?

A. The type of colored glass normally used for glassblowing is not currently manufactured in this country. The colors must be very dense so they do not wash out when they are blown out thin. Both Germany and Italy have long glassblowing traditions and have been making colored glass for hundreds of years. Right now, theirs is the highest quality available. If that changes, so will I.

And yes, it is expensive. I would be very happy if someone in the U.S. began making high quality colored glass for sale at a reasonable price. There have been rumors over the years that this was going to happen, but...

Q. Does glass always come in the form of rods?

A. No, it is molten in the furnace and can be cast, blown, pulled, or handmade into a variety of forms. It is easy to use as, and make into rods, so that is the most common form (for torchwork and for adding color to furnace work). If it will be remelted in a furnace it is usually in the form of cullet, which is chunks of broken glass.

Q. Does it bear any relation to stained glass?

A. Yes, stained glass is colored glass. It can be used as colored glass for glassblowing, but the colors available aren't dense enough for most work. I use it on occassion for special projects.

Q. Did you have a particular reason for wanting to have an electrically heated furnace as opposed to gas fired?

A. Yes, there were several reasons. All else being equal, electric furnaces melt higher quality glass than gas-fired furnaces. They are easier to control - a simple computerized temperature controller as opposed to the mechanical valves and safety shutoff system on a gas furnace. They are quiet - the burner(s) in a gas furnace make a constant roar. And, they are friendlier to the environment than a gas furnace - there are no combustion byproducts that require venting. These are the primary reasons I chose to build an electric furnace.

The main drawback is that electricity is, BTU for BTU, more expensive than gas. While an electric furnace can be built to be more energy efficient than a gas-fired one because there is no exhaust stack for heat to escape from, in most areas an electric furnace is still considerably more expensive to run. The electricity which powers my glass furnace is produced by the hydroelectric plant at Niagara Falls.

Q. Does a knowledge of physics figure into all this?

A. Physics and chemistry, electrical and mechanical knowledge - all of this is useful. I learned more in the first two years of having my own studio than I though I was capable of learning... You can hire someone else with the knowledge, but that is very expensive. And, if you do that, you had better hope he/she is available when something goes wrong. It's far better to know how to do it yourself. In addition, if you will be building your own equipment - you had better know how to weld, too. However, this said, one doesn't need to be a rocket scientist to make glass, but knowledge doesn't hurt.

Q. Can you combine your techniques (e.g., flameworking with molding ) and would you want to?

A. Sure, almost any glass techniques can be combined if you do it right.

Q. I'd like to know if you have to get air bubbles out of glass (like you do with ceramics or they explode in the kiln) and how on earth you can do that.

A. Yes and no. When the glass is originally melted the temperature of the furnace is turned up to decrease the viscosity of the glass so that air bubbles rise to the surface and pop. If there are still bubbles trapped in the glass after that, it isn't a good thing. That's called seedy glass - those tiny bubbles are seeds and are a defect in the glass.

Sometimes, air bubbles are intentionally put into glass as part of the design of a piece. I use air bubbles as a design element in some of my paperweights. These do not weaken the glass (as long as they are not right next to the surface of the piece).

The difference in a ceramic body is that the air is trapped at room temperature and then expands as the piece is fired, cracking the piece. In glass the air is always trapped hot so it can't expand enough to crack anything. Above approximately 1100 degrees F, glass is soft enough to move with any pressure changes bubbles might cause, clay isn't.

Start Page | Navigation Page | Glass Discussion Board

Top of This Page

the URL of this page is: