Accelerated Aging Chamber, Part 1

Q-Lab Corporation, manufacturer of the Q-SUN Xe-3 accelerated aging chamber, promotes this machine as “the simplest, most reliable, and easiest to use full-sized xenon arc chamber available.” Before purchasing ours in February 2014, we began making upgrades to water and electrical systems in our lab to meet its basic requirements. Perhaps naively, we had planned to install the chamber and begin our testing promptly once those upgrades were complete. Throughout following months, we encountered a series of unexpected challenges in the set up and operation of our new chamber. This is the first in a pair of posts that will introduce the Q-SUN Xe-3, its capabilities and some of the theory behind its use, explore the challenges we have had, and suggest some key issues that you might consider when planning to acquire a large piece of new equipment for your laboratory.

THE Q-SUN ACCELERATED AGING CHAMBER

Q-SUN Xe3 Accelerated Aging Chamber with Q-Lab training specialist Alan Boerke for size comparison. AMNH/B. Nunan

PART 1: GETTING UP TO SPEED

Much to the disappointment of some of our curious colleagues in other departments who wondered what one does with an accelerated aging chamber, the Q-SUN Xe-3 can not be used to expedite troublesome developmental phases in your toddler, nor be run in reverse to reunite you with your youth. Too bad. Instead, this machine is used to rapidly reproduce the damage to materials that is caused by light, temperature, and humidity in real environments over longer periods of time.

The tester is a bit bigger than a refrigerator, and contains three powerful xenon arc lamps that expose samples to bright, daylight-imitating light inside of a compartment roughly the size of an oven. The spectrum of light produced can be adjusted with the installation of various filters above the sample compartment. Light output is measured in irradiance (W/m2), and can be controlled at either 340nm (ultraviolet) or 420nm (visible) depending on what filters are in use. The tester also maintains set points for relative humidity, chamber air temperature, and the temperature of a black panel placed inside the sample compartment.

Q-Lab training specialist Alan Boerke discusses the Q-SUN aging chamber with project conservators and other conservation scientists from neighboring NYC institutions. AMNH/B. Nunan

On March 10, 2014 the American Museum of Natural History (AMNH) hosted a training session on accelerated aging and use of our new Q-SUN Xe-3 with Alan Boerke, Technical Sales and Training Specialist at Q-Lab Corporation. The training was attended by selected museum staff and colleagues from Yale’s Institute for the Preservation of Cultural Heritage, the Metropolitan Museum of Art, and the Museum of Modern Art, as well as students from New York University’s Institute of Fine Arts Conservation Center.

Accelerated aging makes use of the principle that exposure to high intensity light for a short time can produce deterioration similar to that caused by low intensity light over a longer time. However, in order to correctly interpret one’s results, one must understand that for many reasons, accelerated aging does not occur in a way that is strictly reciprocal. In part this is due to the inability of any aging chamber to exactly replicate every aspect of real-world exposures: wet/dry, thermal, or light/dark cycling, the spectrum of incident light, and the presence of air pollutants, dust, or adjacent materials may be impossible to simulate. This non-equivalence is also a consequence of thermal chemistry that unfolds simultaneously alongside light damage, but can’t easily be differentiated from it or accelerated proportionally.

Alan emphasized benchmarking as a way of managing this problem. To create a benchmark, materials aged in real-time are used to define the mode and extent of change taking place over a known duration. When a comparable degree of change is observed in the accelerated test, a correlation factor can be identified to be used in calculating an approximate relationship between accelerated and real-time aging. However, benchmarking has some obvious drawbacks, not the least of which is that a material that ages well may take many many years to fail in a real-world exposure environment. If one is conducting accelerated aging on that material, it’s usually because one needs information promptly and can’t afford to wait.

The success of this approach depends on the selection of a standard that exhibits deterioration behavior similar to the samples being tested- both in the real world and accelerated aging environments.  However, since the samples being tested have unknown aging behaviors one standard is usually insufficient.  So instead of choosing a single standard, it is better to select a series of standards that will hopefully bracket the behavior of the samples.  For lightfastness testing, a common set of standards is the Blue Wool scales, wool swatches dyed with eight different dyes that exhibit a range of different lightfasnesses.  By including the Blue Wool scales in our accelerated tests we can determine which of the eight standards our samples behave most similarly to.

Blue Wool scale assembled by the team using blue wool reference standards 1-8 obtained from SDC Enterprises Lmtd and mounted onto card stock. A new Blue Wool reference scale will be used with each test round. AMNH/F. Ritchie

Our training session also included a discussion of other factors that could affect the results of our testing: the color and cleanliness of the sample, whether it is mounted at an angle, or over a backing board, and its height inside the sample compartment; variations in sample handling and measurement technique; breaks in our test cycle for sample measurement; and the age of the xenon lamps. Getting repeatable results hinges on limiting variation in these influences.

We concluded with a tutorial in which Alan showed us all the basics for running and maintaining the machine: how to load and rotate samples, install and calibrate lamps, change light filters, and program the desired parameters.

With all of this new knowledge in hand, we promptly began the process of translating our research plan into an actual method for mounting and testing our dye samples. Very quickly we observed that doing so would not simply be a matter of plugging in the Q-SUN, programming the ASTM D4303 test parameters, and pressing the ON button. Many unanticipated challenges were yet to come…

Conservation intern Ersang Ma prepares to calibrate the Q-SUN lamps, according to procedures learned during the one-day training session by Q-Lab. AMNH/B. Nunan