[an error occurred while processing this directive] Volume 15, Number 2, May 1993, pp.12-14
Ageless is the tradename of an oxygen scavenger patented and produced by the Mitsubishi Gas Chemical Company1,2. It is available in several different compositions, since its primary commercial use is in food packaging, and foods vary in moisture and carbon dioxide content. We conducted our studies on packets of Ageless-Z, stated by Mitsubishi to be a mixture of finely divided moist iron (ferrous) oxide and potassium chloride. Ageless-Z is designated as Z-100, Z-1000, etc., to indicate the milliliters of oxygen with which a single packet will react.
This article will illustrate the practical aspects of using Ageless with examples and several notes of caution. Conservators interested in the experimental details and analysis can refer to our publication in Studies in Conservation3.
We conducted experiments to determine three important properties: (1) the effect of Ageless capacity and chamber or case volume on the oxygen concentration; (2) the effect of exposure on the reaction rate of Ageless; and (3) the effect of low relative humidity on the reaction rate of Ageless.
The rate of absorption of oxygen by Ageless is proportional to the Ageless capacity in liters divided by the case volume in liters (denoted as "absorbance" henceforth). For example, two packets of Ageless Z-2000 in a 100-liter case would have an absorbance of 0.04 (since each Ageless Z-2000 packet has an oxygen capacity of 2000 ml or 2 liters, the total Ageless capacity is 4 liters, which is divided by the chamber volume of 100 liters). Decreasing the volume of the case to 50 liters or doubling the number of Ageless packets to four would double the absorbance and hence double the rate of oxygen absorption.
One set of experiments that we conducted evaluated the effect on the reaction rate if only one side of the flat Ageless packets was readily accessible to the atmosphere, such as when packets are laid on a case floor. We found the rate of oxygen reaction with Ageless was 20% less in this position than when packets were elevated on a wire support that exposed both sides. This 20% rate difference with Ageless on the case bottom rather than on a porous support need not concern the conservator in practice. Even a 100% difference would mean only that an equilibrium level of oxygen in a case over a year or multi-year period would be 80 ppm rather than 40 ppm or 160 ppm rather than 80 ppm. Similarly, the expected differences in rate between batches of a commercial product such as Ageless are immaterial to a practicing conservator who normally would use a considerable excess of Ageless. The oxygen capacity of the Ageless is important and is reported to be carefully controlled by Mitsubishi, for obvious economic competitive reasons.
Mitsubishi recommends a minimum of 55% RH for using Ageless-Z. When packets of Ageless (2000 ml of oxygen capacity) were placed in a sealed 38-liter chamber maintained at 33% RH with saturated magnesium chloride solution, the reaction rate remained constant for four months but then dropped to one-seventh of that rate by the end of one year. This would be of importance for long-term preservation of materials requiring low-RH environments. (Recently a particular type of Ageless has been developed by Mitsubishi for low relative humidity applications.)
An obvious use of a material such as Ageless is to extend the life of a sealed case or a bag filled with an inert gas by reacting with any oxygen that leaks into it. With Ageless, the need for flushing the case again with inert gas because of oxygen ingress undoubtedly can be postponed by years if the leak rate is not too high. Using the rate of reaction of oxygen with Ageless established by our research3, calculation of the equilibrium level of oxygen in a sealed but leaking case containing Ageless is straightforward. Any oxygen leaking into the case must immediately react with the Ageless; i.e., the leak rate cannot be greater than the rate of reaction of Ageless with oxygen. Based on this, we derived a simple equation:
[O2] = L / 12.7 C (Equation 1)
Where "L" is the leak rate of the case or bag in ppm per day, "C" the absorbance, and O2 the equilibrium oxygen concentration. Hence knowing the case volume and its leak rate per day, the required Ageless capacity (in liters of oxygen) can be found for any desired oxygen level.
Now for some recommendations and notes of caution. Where five or more Ageless Z-1000 packets are required, it is easier to use the Z-2000 units. After removal from the protective shipping package, either size of packet can be quickly stapled with others on their margins to form a chain or strip which can be slid into the case rapidly while nitrogen flows out, and be readily exchanged with a new strip of packets when replacement is necessary. Alternatively, the Ageless packets can be placed edges-up in rows on a tray if separated by strips of expanded metal to permit ready atmospheric access to the packet sides. A few minutes' exposure to air is acceptable; more than ten minutes, however, is unwise because of the beginning of exothermic warming and also the loss of 0.02% of the oxygen-absorbing capacity per minute.
Determination of the leak rate of a sealed case in which Ageless is to be placed is essential for predicting the time span it will be effective, or the equilibrium oxygen value. The procedure for determining the leak rate involves flushing the case with nitrogen and monitoring the concentration of oxygen as a function of time. Care needs to be taken in choosing the right oxygen measuring instrumentation. The oxygen-measuring apparatus can consist of a passive sensor in a case or bag and a detector sensitive to parts per million of oxygen.
A major reason for the use of sealed cases and other containers filled with inert gas such as nitrogen is prevention of oxidation in the objects placed therein. To prevent such oxidation, much work has been reported of sealing objects in plastic pouches with Ageless. In small flexible containers with little air content, Ageless can serve well despite some warming. However, it is hazardous to place Ageless in a large rigid case containing air, both because of the heat produced and because of the danger of implosion when the oxygen, 20% of air, is removed by reaction with Ageless. A sealed case filled with an inert gas in a museum (rather than in a more controlled laboratory environment) must have a flexible bellows attached to compensate for temperature and pressure fluctuations in the museum atmosphere.
Also, since the reaction of Ageless with oxygen is exothermic, and the Ageless packets tend to become very hot, Ageless should never be placed directly on an object. Studies have also indicated that the relative humidity in the proximity of Ageless increases as the packets absorb oxygen4.
Ageless absorbs an amount of carbon dioxide equal to its oxygen capacity5,6, hence conservators using Ageless during carbon- dioxide fumigation should be aware of this fact.
The use of Ageless will be illustrated with two examples.
The problem: To maintain a hermetically sealed 500-liter display case designed for the royal mummies in Egypt for 10 years at less than 1000 ppm (0.1%) oxygen concentration. The leak rate was accurately determined to be 10 ppm/day (0.001%). By purging with pure nitrogen, the initial oxygen concentration has been reduced to zero.
Step 1: Converting the leak rate from ppm to ml of oxygen per day=10/1,000,000 x 500 liters/1 x 1000 ml/1 liter=5 ml/day
Step 2: Calculate the amount of oxygen leaking over a ten year period: 10 x 365 x 5=18250 ml or 18.25 liters.
Step 3: Calculate the number of Ageless Z-2000 packets required: Because each packet can absorb 2000 ml or 2 liters, 10 packets should be used to absorb 18.25 liters.
Step 4: Calculate equilibrium oxygen content for 10 packets of Ageless Z-2000: This involves two steps:
a) The absorbance "C" = oxygen-absorbing capacity of the Ageless in liters/case volume in liters=20/500=0.04.
b) The equilibrium oxygen content using equation (1) = L/12.7 C where "L" the leak rate is 10 ppm/day. Equilibrium oxygen concentration=10/12.7 x 0.04=10/0.508=19.6 ppm. This is well within the 1000 ppm (0.1%) that we desire to maintain.
The problem: To maintain the oxygen concentration in a 1000-liter bag at less than 2000 ppm (0.2%) for a period of 14 days. The leak rate was accurately evaluated to be 250 ppm/day (0.025%). By purging with nitrogen the initial oxygen concentration has been reduced to zero.
Step 1: Converting the leak rate in ppm to ml of oxygen per day=250/1,000,000 x 1,000 liters/1 x 1000 ml/1 liter=250 ml/day.
Step 2: Calculate the amount of oxygen leaking over a 14-day period: 250 x 14=3500 ml or 3.5 liters.
Step 3: Calculate the number of Ageless Z-2000 packets required: Because each packet can absorb 2000 ml or 2 liters, 2 packets will absorb 3.5 liters.
Step 4: Calculate the equilibrium oxygen content for 2 packets of Ageless Z-2000:
a) The absorbance "C" = oxygen absorbing capacity in liters/case volume=4/1000=0.004
b) The equilibrium oxygen content using equation (1) = L/12.7 C where "L" the leak rate is 250 ppm/day.
Equilibrium oxygen concentration=250/12.7 x 0.004=250/0.0508=4921 ppm. Since this is greater than the 2000 ppm (0.2%) that we desire to maintain, we will have to increase the number of Ageless packets. By repeating Step 4 for different number of Ageless packets, we find that 5 packets of Ageless Z-2000 can maintain an equilibrium oxygen concentration of 1968 ppm which is below the desired level.
In practice, as for Example 1, we would advise doubling the number of Ageless packets as calculated above.
We express our sincere thanks to Shin Maekawa and Frank Preusser from the Getty Conservation Institute for their valuable suggestions and support, and to Dr. Mark Gilberg for introducing this product to the conservation community. We also thank Jim Davies from the Getty Conservation Institute for his creativity in the machine shop.
Ageless: Conservation Materials, Ltd., P.O. Box 2884, Sparks, Nevada 89432
1. "Ageless Oxygen Absorber: A New Age in Food Preservation," brochure from Mitsubishi Gas Chemical Company.
2. United States Patent No. 4,127,503. Nov. 28, 1978.
3. Lambert, F.L.; Daniel, V.; Preusser, F.: "The Rate of Absorption of Oxygen by Ageless: The Utility of an Oxygen Scavenger in Sealed Cases," Studies in Conservation, Vol. 37, pages 267-274 (1992).
4. Personal communication with John Burke, Oakland Museum, Oakland, California.
5. Personal communication with Yukio Kondoh, Mitsubishi International Corporation, New York.
6. Personal communication with Mark Gilberg, Nicasio, California.
address for correspondence:Vinod Daniel
Vinod Daniel has been a senior fellow in environmental sciences at the Getty Conservation Institute since 1991. He has master's degrees in chemical engineering (with concentration in polymers) and physical chemistry. His areas of interest include nontoxic fumigation, moisture buffers, museum cases, and environmental monitoring.
Frank L. Lambert received his PhD from the University of Chicago. He is professor emeritus of chemistry at Occidental College where his research was mainly in physical organic chemistry. He has served as a consultant to the J. Paul Getty Museum and the Getty Conservation Institute since 1982.
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