One of the health concerns I have as a potter is protecting my eyes from the dangers of looking into a red-hot kiln when checking pyrometric cones. During one of my pottery classes, we were discussing this potential danger and one of the potters present just happened to be an eye doctor, Laura T. Muller, MD. She offered to share some important information to help us all be more aware of eye safety.
Glenn Woods: How could a glowing hot kiln (1) hurt my eyes?
Laura T. Muller, MD: To understand the potential harmful ocular effects from a glowing kiln (either gas or electric), it is helpful to know the difference between ultraviolet light, visible light, and infrared light (2). Ultraviolet light has the shortest wavelength; visible light has a mid-range wavelength; and infrared light (the source of damage from looking into kilns) has the longest wavelength. Infrared light, which is also known as infrared radiation, can be experienced as heat and these invisible infrared wavelengths can damage the eye.
When looking at a glowing kiln, potters suppress their aversion response, which typically would cause you to blink and/or turn your head away within 0.25 seconds of looking at a bright hot light. Suppressing this aversion response can put the eye at risk for injury.
GW: What parts of the eye can be injured?
LM: First let’s review of the anatomy of the eye (3). The cornea is the window in the front of the eye. The lens is located behind the iris (the colored part of the eye that makes blue eyes blue and brown eyes brown). The lens is the most susceptible to infrared damage from a glowing kiln. Damage to the lens can lead to cataract formation and cloudy vision. The retina is the film in the back of the eye that can also be injured by prolonged gazing into the glowing kiln, with the result being blind spots or portions of vision disappearing.
GW: At what point during the firing is it dangerous to look into the kiln?
LM: Infrared radiation starts at 752°F (400°C). Looking at a kiln at any temperature above this should always be done with eye protection.
GW: Can I just use sunglasses or glassworker’s glasses to look into the glowing kiln?
LM: Neither of these options provide protection from infrared light. Used by glassworkers, traditional didymium glasses are almost perfectly transparent allowing the glassworker to see the subject more clearly. Without additional treatments, they do not offer protection from infrared radiation. Sunglasses may protect from UV-A and UV-B, but do not offer any protection in the infrared range.
GW: What eyewear can potters use to protect their eyes from the glowing kiln?
LM: Personal protective equipment (PPE), in the form of safety eyewear designed to block infrared radiation is needed when looking into a glowing kiln.
There are several materials that could be used in eyeglasses for blocking out infrared light. Reflective metallic coatings are particularly effective at preventing transmission of infrared light. Aluminum and Inconel (an alloy of iron, nickel, and chromium) are a good choice but not as readily available. Pfund’s glass is a gold-plated glass that blocks 99% of infrared rays emitted at temperatures over 1922°F (1050°C). Most people know of this as the gold coating used in astronaut helmets to protect them from infrared radiation in space.
Welder’s glasses are a good choice to block out infrared light. Choosing a level that allows you to comfortably see the cones is ideal. One potter I spoke with used a level 8 shade. All well-constructed quality welding lenses have a screen that filters out 100% of the harmful ultraviolet (UV) and infrared (IR) wavelengths and provides protection to the eyes. The shade number denotes the amount of darkness provided by that particular lens and should be used as a guide to select the one that is most comfortable and yet provides good visibility for the
There are also ANSI Z87.1 safety glasses available specifically to block the infrared range. The markings on eye protection relate directly to the equipment’s ability to defend against specific hazards. For infrared radiation, the designation is the letter R. Honeywell makes the S3430X Flex Seal Goggle—a great choice for eye protection that fits over glasses and allows for good visualization of the cones (4).
GW: As a potter, I feel the need to look inside the glowing kiln. What are other ways to monitor the firing progress?
LM: Wearing the proper eyewear can allow a potter to look into a glowing kiln with minimal risk. However, to avoid this risk completely, kiln setters and digital controllers are generally reliable for electric kilns. Also, when using electric kilns, a baby monitor placed in the kiln room can help to see the temperature on the control panel and hear the relays clicking or alarms sounding. Witness cones can still be used to confirm what happened after the firing. This may not be as practical for potters who use gas-fired kilns.
Protecting your eyes with the proper eyewear costs less than $25 and can help minimize the risk of eye damage and contribute toward healthy vision.
Laura T. Muller, MD is a board-certified ophthalmologist and fellowship-trained specialist in cornea, external disease, and refractive surgery. She owns and operates at The Eye Center in Oldsmar, Florida. Learn more at www.lauramullermd.com.
Glenn Woods and his husband, Keith Herbrand (AKA Pottery Boys Clay Studios), make their home and studio in Palm Harbor, Florida, and Blue Island, Illinois. Woods also teaches at the Dunedin Fine Art Center in Dunedin, Florida. Learn more at www.PotteryBoys.com.