Introduction
\nMuseums are faced with the challenge of preserving artifacts for the enjoyment and education of current and future
\ngenerations. The wide variety of artifacts that museums display requires maintaining specific environmental
\nconditions to minimize their deterioration. Proper relative humidity (RH) control is one of the most important
\nenvironmental factors in artifact preservation.
\nConservation research has shown that RH levels above 65% will promote microbial growth (primarily fungi), while
\nRH levels below 25% can lead to brittleness and cracking. In addition, large fluctuations in RH can lead to
\ndimensional changes, deformation, and mechanical stress in organic materials. Though there is still debate about the
\nappropriate RH requirements for museum environments, a set point of 50% (or the historic building average) with
\nallowable fluctuations of \u00b15-10% is a generally accepted guideline.
\nThough mechanical humidification systems for RH control are common, many museums do not use them due to
\nfactors such as cost and the difficulty integrating them with the existing building structure and aesthetics. An
\nalternate method of RH control that is employed in many museums is the use of an adsorbent material in
\ncombination with a well-sealed display case to create a micro-environment that serves to mitigate the large RH
\nfluctuations that the general building environment may experience.
\nThe most commonly used adsorbent is silica gel due to its high water capacity, chemical inertness, and ability to
\nundergo an indefinite number of moisture cycles. This method has been prescribed in the conservation literature for
\nmany years, most notably by Thomson (1977) who developed the idea of ‘hygrometric half-time,’ t1\/2, as the time it
\ntakes for the RH inside a case to reach the halfway point of the ambient RH. The hygrometric half time is calculated
\nby the following equation:
\nThomson calls M the ‘specific moisture reservoir’ of the buffering material. M is defined as the mass of water (grams
\nof water) gained or lost per unit mass of adsorbent media (one kilogram of buffering material) for a 1% change in
\nRH. The M-value is essentially the slope of the buffering material’s adsorption isotherm evaluated at a specific RH,
\nand it describes the incremental moisture buffering capacity of a material. In order to provide effective RH control,
\nthe adsorbent material must have a high M-value over the acceptable RH range for a given artifact. Increasing the Mfile:\/\/\/
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\nAn Evaluation of Silica Gel for Humidity Control in Display Cases
\nvalue reduces the amount of adsorbent required to achieve proper control.
\nAccording to Thomson, the hygrometric half time is dependent upon two main factors: (1) the air leakage rate of the
\ncase, N and (2) the amount of the buffering material inside the case, B. B will be referred to as the gel sizing factor; it
\nis the dry mass of buffering material in kg per cubic meter of case volume. By decreasing the air leakage rate or
\nincreasing the amount of buffering material, the hygrometric half time is increased, and greater RH control inside the
\ncase can be achieved.
\nThomson states that a well-sealed case can be expected to have an air leakage rate of one air change per day. Using
\nEq.1 Thomson calculates that 20 kg\/m3 (1.25 lb\/ft3) of silica gel with an M-value of 2 g\/kg would be needed to
\nachieve a hygrometric half-time of 150 days for a display case having a leakage rate of 1 air change per day (ACD).
\nThis recommendation has become the standard guideline for using silica gel as a passive RH control method inside
\ndisplay cases.
\nPrevious studies, e.g., Guinchen and Gai (1984), Schweizer (1984), Stolow (1977), have found silica gel to be an
\neffective method of RH control inside display cases. However, in a recent research project evaluating the
\nenvironmental conditions at the Field Museum of Natural History in Chicago a silica gel application was found to be
\nineffective at controlling the RH inside a newly-constructed display case.
\nThe case had a volume of 1.7 m3 (60 ft3) and contained Chinese wood and ivory carvings. Three cassettes, each
\ncontaining 750 grams (1.65 lbs) of Art-Sorb, were located in drawers at the bottom of the case. The Art-Sorb was
\nconditioned to a relative humidity of 45% prior to being placed inside the case. Figure 1 shows the relative humidity
\nlevels measured inside and outside the case over a two-month period from February to April of 1999. The Art-Sorb
\nhad no discernible buffering effect with the relative humidity levels inside the case being almost the same as the
\nambient levels and well below the desired level of 45%.
\nFigure 1. Comparison of RH levels inside a display case containing Art-Sorb with ambient building levels at the
\nField Museum of Natural History in Chicago, Illinois.
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\nAn Evaluation of Silica Gel for Humidity Control in Display Cases
\nThis experience became a motivating factor for the present study. The goal of this study was to evaluate the
\neffectiveness of silica gel for use in controlling RH levels inside display cases. The study consisted of the following
\nparts:
\nm experimentally determine the adsorption and desorption isotherms of the three silica gels
\nm develop a mathematical model of the solid adsorbent media for passive RH control in a display case
\napplication
\nm experimentally validate the model
\nm use computer simulations to evaluate the effect of varying building and case conditions on silica gel
\nperformance.
\nThree silica gels were evaluated in the study. They were generic regular density silica gel, which is available from
\nmany chemical supply companies, and two specialty gels marketed specifically for museum conservation
\napplications: Artengel and Art-Sorb.
\nExperimental Measurement of Adsorption and Desorption Isotherms
\nThe adsorption and desorption isotherms describe the equilibrium moisture concentration (EMC) of a material at
\ndifferent RH levels.
\nThe isotherms of the three gels were measured experimentally by allowing samples to come into equilibrium at
\ndifferent RH levels inside a constructed humidity chamber and measuring their weight gain in water. Equilibrium
\nwas attained when the measured weight of the gel samples did not change over time.
\nTable 1. Equilibrium relative humidity values at
\n25\u00b0C. for salt solutions used in isotherm
\nexperiments from Greenspan (1977).
\nSalt Equilibrium RH % at 25
\nC
\nLithium Chloride 11
\nMagnesium
\nChloride
\n33
\nPotassium
\nCarbonate
\n43
\nSodium Bromide 58
\nSodium Chloride 75
\nThe isotherms were measured three times to verify their accuracy. Weight measurements were made with a Sartorius
\nL420 S top-loading laboratory scale (accuracy \u00b10.0005 g) which was located in the humidity chamber. Different RH
\nlevels were achieved using the saturated salt solutions listed in Table 1, the work of Greenspan (1977).
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\nAn Evaluation of Silica Gel for Humidity Control in Display Cases
\nThe RH inside the chamber was measured using a Hobo H8 Pro Series Temp\/RH data-logging sensor with a stated
\nmanufacturer’s accuracy of \u00b13% RH and \u00b10.3\u00b0C. An isotherm set was also measured using a high accuracy General
\nEastern Hygro M-1 dew point hygrometer (accuracy \u00b10.2\u00b0C).
\nExperimental Isotherm Data
\nTable 2 shows the experimental adsorption and desorption isotherm data for the three gels. All the data were
\nobtained at temperatures between 23-25\u00b0C. The EMC of each gel is expressed as a percentage of the gel’s dry
\nweight, so an EMC of 20% means that the gel would be able to hold an amount of water equal to 20% of its dry
\nweight.
\nAdsorption Isotherm
\nRH % Equilibrium Moisture Concentration(%)
\nRegular
\nDensity
\nArtengel Art-Sorb
\n11 9 8 8
\n11 9 9 9
\n11 7 7 8
\n33 19 17 14
\n33 20 19 16
\n33 19 18 15
\n44 24 23 18
\n44 25 24 19
\n44 26 25 19
\n60 31 35 25
\n60 32 35 27
\n63 32 37 31
\n74 34 43 53
\n74 34 43 54
\n100 37 48 95
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\nAn Evaluation of Silica Gel for Humidity Control in Display Cases
\n100 37 48 100
\n100 37 47 109
\nDesorption Isotherm
\nRH % Equilibrium Moisture Concentration (%)
\nRegular
\nDensity
\nArtengel Art-Sorb
\n11 9 9 10
\n11 8 8 8
\n11 9 8 9
\n34 24 20 15
\n34 24 20 17
\n34 25 21 17
\n46 31 33 21
\n46 31 34 20
\n46 31 35 22
\n62 33 41 31
\n62 33 41 34
\n65 33 41 39
\n74 34 43 66
\n74 34 44 66
\nThe experimental isotherm data were fitted to the Dubinin-Astakhov (D-A) (1977) equation using a non-linear least
\nsquares method. The D-A equation expresses the equilibrium moisture concentration of the gel as a function of the
\nadsorption potential, A defined as:
\nThe Dubinin-Astakhov equation is given by:
\nEMC%=q x 100 (Eq. 4)
\nwhere q0, q1, E0, E1, n1, and n2 are equation parameters.
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\nAn Evaluation of Silica Gel for Humidity Control in Display Cases
\nFigures 2-4 compare the isotherm’s curve fits from the Dubinin-Astakhov equation with the experimental data for
\nregular density silica gel, Artengel, and Art-Sorb, respectively. All three gels show some hysteresis with the
\ndesorption isotherm lying above the adsorption isotherm. Art-Sorb has the least amount of hysteresis with most of it
\noccurring above 60%. Regular density silica gel’s hysteresis occurs below 60%, while Artengel’s occurs in the 30-
\n70% range.
\nFigure 2. Regular density silica gel.
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\nAn Evaluation of Silica Gel for Humidity Control in Display Cases
\nFigure 3. Artengel
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\nAn Evaluation of Silica Gel for Humidity Control in Display Cases
\nFigure 4. Art-Sorb
\nGel Comparison
\nIn comparing the gel isotherms, it is best to focus on the 30-60% RH range since that is the desirable range for
\nmuseum environments. Figure 5 shows the EMC of the three gels in the 30-60% RH range. In this range, Art-Sorb
\nhas a lower moisture capacity than Artengel and regular density silica gel, which have similar capacities.
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\nAn Evaluation of Silica Gel for Humidity Control in Display Cases
\nFigure 5. Equilibrium moisture capacity of three gels in the relative humidity range of 30-60%.
\nIn evaluating the buffering performance of the three gels, it is more appropriate to use the change in moisture
\ncapacity over a given RH range, represented by the M-value, rather than the specific moisture capacity at a specific
\nRH as shown on the isotherm plots. Over a given RH range, a gel with a linear isotherm will have a constant Mvalue
\nand thus a constant performance, while a gel with a non-linear isotherm will have a varying M-value and thus
\nits performance will vary with RH. Since the M-value is essentially the slope of the isotherm at a given RH, it can be
\ncalculated for each of the three gels by taking the derivative of the D-A equation. Figure 6 compares the M-values
\nfor the three gels based upon their adsorption isotherms over the 30-60% RH range. Artengel has a fairly constant Mvalue
\nof 7 g\/kg. The M-value for Art-Sorb increases with RH from 3 to 10 g\/kg, while regular density’s M-value
\ndecreases with RH from 6 to 3 g\/kg.
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\nAn Evaluation of Silica Gel for Humidity Control in Display Cases
\nFigure 6. Comparison of M-values for the three adsorbants in the 30-60% RH range.
\nBased upon these results, Artengel would be expected to have a consistent performance over the 30-60% RH range.
\nArt-Sorb’s performance would be better in the higher RH range, while regular density’s performance would be better
\nin the lower RH range.
\nUsing eq. 1, Thomson’s original calculations were revisited using the present results to determine the amount of each
\ngel required to achieve a case half-time of 150 days with a case leakage rate of 1 ACD. Table 3 lists these results
\nalong with the calculated cost of each gel per unit of case volume.
\nTable 3. Comparison of calculated gel sizing factors and costs
\nusing Thompson’s equation to achieve a case half-time of 150
\ndays with a case leakage rate of 1 ACD.
\nGel Gel Mvalue
\n@
\n45%
\nRH
\nGel
\nSizing
\nFactor
\n(kg gel\/
\nm3 of
\ncase)
\nGel
\nCost ($\/
\nkg of
\ngel)
\nGel
\nCost ($\/
\nm3 of
\ncase)
\nRegular density 4.5 7 16 110
\nArtengelTM 7 4.6 22 100
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\nAn Evaluation of Silica Gel for Humidity Control in Display Cases
\nArt-Sorb TM
\n(study data)
\n3 10 40 400
\nArt-Sorb TM
\n(company data)
\n4.7 6.5 40 260
\nThe results presented in Table 3 are based upon each gel’s M-value at 45% RH (the middle of the 30-60% range). Art-
\nSorb’s M-value was calculated using both the measured isotherm data and the data listed in the company’s literature.
\nRegular density silica gel costs are calculated from the stated bulk price from a common manufacturer. The costs for
\nArtengel and Art-Sorb are based upon each company’s stated price for bead type media.
\nSince Artengel has the highest M-value of 7 g\/kg, it has the smallest sizing factor of 5 kg\/m3. Art-Sorb and regular
\ndensity silica gel have about the same M-value around 5g\/kg and thus have similar sizing factors of around 7 kg\/m3.
\nOn a cost basis, Artengel and regular density have nearly the same unit cost (i.e. cost per unit of case volume), while
\nArt-Sorb has more than double the cost of regular density silica gel.
\nCase Model
\nIn order to evaluate the long-term performance of the three gels under varying conditions, a computer model was
\ndeveloped. The following assumptions were used in the modeling of silica gel behavior inside of a display case:
\nm Case material and artifact have negligible water buffering capacity i.e. the buffering material preferentially
\nadsorbs and desorbs moisture.
\nm No spatial variation of RH inside the display case.
\nm Dry bulb temperature inside the case microenvironment is the same as the building macroenvironment.
\nm Temperature of the buffering media is the same as the dry bulb temperature in the case microenvironment.
\nThe two water exchange processes included in the model are: (1) the exchange of water vapor between the case
\nmicroenvironment and ambient building macroenvironment air due to infiltration and (2) the exchange of water
\nvapor between the gel and case air. The rate of water exchange due to infiltration is expressed by:
\nBecause the movement of air inside a well-sealed display case is dominated by free convection, intra-particle
\nresistance is assumed to be negligible and the transport of water vapor from the bulk case air to the gel surface is
\nassumed to be the dominant resistance to mass transfer. From the work of Kafui (1994), the rate of mass transfer
\nfrom the case air to the gel surface can be described by a lumped external mass transfer coefficient, Kgel, with the
\nchange in water concentration of the adsorbent media with time being expressed by:
\nA water balance that includes these two water exchange processes can be written
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\nAn Evaluation of Silica Gel for Humidity Control in Display Cases
\nwhere mH2O is the mass of water in the case at time t and mH2O,0 is the initial mass of water. Knowing mH2O, the
\nrelative humidity can be calculated as a function of time using psychrometric relations.
\nModel Validation
\nThe validity of the model was determined by comparing model predictions with the results of a long-term experiment
\nthat measured the RH inside three moisture-impervious containers, each with a volume of 3.79 liters.
\nEach container held a 10-gram sample of one of the three gels which was initially conditioned to an RH of 43%. A
\n30-mm hole located in the top of each container allowed each container to have a similar leakage rate. The RH inside
\nthe containers and the ambient room value was measured using a Hobo H8 Pro Series Temp\/RH sensor. The longterm
\nexperiments involved monitoring the RH for a period spanning 42 days.
\nFigure 7 compares the model predictions with the long-term experimental data for regular density silica gel. Figure 8
\ncompares the performance of the three gels for the long-term experiment. Regular density silica gel and Artengel
\nperform almost identically with a final RH of 26%, while Art-Sorb is less effective with a final RH of 20%. For all
\nthree gels, the model prediction is within the \u00b13% accuracy of the measured data.
\nFigure 7. Comparison of model prediction with experiment data for regular density silica gel.
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\nAn Evaluation of Silica Gel for Humidity Control in Display Cases
\nFigure 8. Comparison of gel performance for long-term experiment.
\nSimulation of Silica Gel Performance
\nUsing the model, simulations were run to evaluate the performance of the three gels inside display cases under
\nvarying building and case conditions. Three different ambient building RH profiles representing a dry, humid, and
\nmoderate building environments were used to drive the model. A dry building environment was simulated by using
\nRH measurements taken over a one-year period at the Field Museum of Natural History. The 7-month heating season
\nfrom October to April results in an average annual building RH of 35% with the RH dropping below 40% for 60% of
\nthe year. A humid building environment was simulated by inverting the Field Museum data to create an average
\nannual building RH of 55%. A moderate building environment was represented by a sine wave with an average RH
\nof 45% and an amplitude of 15%.
\nThe three variables analyzed with the model were the case leakage rate, amount of gel inside the case, and the type of
\ngel. For Art-Sorb, simulations were run using both the company’s adsorption isotherm data and the isotherm
\ndetermined from the current study’s experiments. Table 4 lists the parameters and values that were used in the
\nsimulations.
\nCase model simulation parameters and values.
\nSimulation Parameter Values Used in Simulations
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\nAn Evaluation of Silica Gel for Humidity Control in Display Cases
\nGel Sizing Factor (kg\/m) 5, 10, 15, 20, 30, 50
\nCase Leakage Rate (ACD) 0.25, 0.5, 1, 2,5
\nCase RH set point 45%
\nInitial Gel RH Building
\nEnvironment
\n45%
\ndry, humid, moderate
\nSimulation Results
\nFigures 9-11 compare the simulation results for the dry, humid, and moderate building RH environments,
\nrespectively. The Y-axis on each plot is the maximum predicted RH fluctuation inside a case over a one-year
\nsimulation. The X-axis is the amount of gel inside the case per unit of case volume in the units of kg\/m3. The
\ndifferent lines on each graph represent the performance of the gel at a specific case leakage rate value, N, in units of
\nair changes per day. For example, looking at the Artengel plot in Fig. 9, a case with a leakage rate of 2 ACD (n=2)
\nand containing 20 kg\/m3 of gel would undergo an RH fluctuation of 8% from the desired set point of 45% over an
\nentire year in a dry building environment.
\nFor all three building environments, Artengel has the best performance yielding smaller case RH fluctuations than
\nthe other two gels for a given case leakage rate and gel sizing factor. Since it has a constant M-value over the 30-
\n60% RH range, Artengel’s performance is not affected by different building environments. Since its M-value
\nincreases with RH, Art-Sorb performs better in a humid building than a dry one. In contrast, regular density silica gel
\nperforms better in a dry building than a humid one because its M-value is highest in the lower RH range.
\nThere is little difference in the predicted performance of Art-Sorb using either the company or current study’s
\nisotherm data. The amount of gel required to keep the RH fluctuation to an acceptable level increases as the case
\nleakage rate increases. If the maximum desired case RH fluctuation over a one-year period is 10%, then a case
\nleakage rate of 2 ACD or less is necessary. Higher case leakage rates would require more than 30 kg\/m3 of gel which
\nwould likely be impractical and costly. Within the case leakage range of 0.25-2 ACD, between 5-30 kg\/m3 of silica
\ngel would be required for adequate control over a one-year period.
\nFigure 9.Maximum predicted case RH fluctuations over one-year simulation for three silica gels in a dry
\nbuildingenvironment for different case leakage rates (N) and gel sizing factors
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\nAn Evaluation of Silica Gel for Humidity Control in Display Cases
\nFigure 10.As above, for humid buildingenvironment
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\nAn Evaluation of Silica Gel for Humidity Control in Display Cases
\nFigure 11.As above, for moderate buildingenvironment
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\nAn Evaluation of Silica Gel for Humidity Control in Display Cases
\nConclusions
\nBased upon the experimental and computer simulation results of this study, the following conclusions can be made:
\nm All three gels would provide effective RH control using Thomson’s recommendation of 20 kg\/m3 if the case
\nleakage rate is 1 ACD or less.
\nm On a pound for pound basis, Artengel has a higher buffering capacity than either regular density silica gel or
\nArt-Sorb.
\nm On a cost basis, regular density silica gel and Artengel have about the same cost for a given amount of
\nbuffering capacity while Art-Sorb has twice the cost.
\nm Over the RH range of 30-60%, Artengel has a consistent performance, while Art-Sorb performs better above
\n50% and regular density silica gel performs better below 40%.
\nm For the conditions investigated, a case with a leakage rate of greater than 2 ACD would require more than 30
\nkg\/m3 of gel in order to keep the display case RH fluctuation at 10% or below over a one year period.
\nm For the conditions investigated, a case with a leakage rate of 2 ACD or less would require 5-30 kg\/m3 of gel
\nto keep display case RH fluctuations to 10% or less over a one year period.
\nReferences
\nASHRAE, Applications Handbook, American Society of Heating, Refrigerating, and Air Conditioning Engineers,
\nAtlanta, GA (1999).
\nAult, J., S.A. Klein, D.T. Reindl, J. Guay, “Indoor Environmental Control: Review of Current Recommendations and
\nSurvey of Conditions at the Chicago Field Museum,” accepted for ASHRAE Transactions, March, 2001.
\nCCI (1984). Technical Bulletin 10: Silica Gel. CCI. Ottawa, Canada.
\nDubinin, M., “Physical Adsorption of Gases and Vapors in Micropores,” Prog. Surf. Membrane Science, 9, pp. 1-70,
\n(1975).
\nErhardt, D. and Mecklenburg, M., “Relative Humidity Re-examined,” Preventive Conservation: Practice, Theory and
\nResearch, ed. by A. Roy and P. Smith. London, IIC, pp. 32-38, (1994).
\nGreenspan, L., “Humidity Fixed Points of Binary Saturated Aqueous Solutions,” Journal of Research of the National
\nBureau of Standards – A. Physics and Chemistry, Vol. 81A, No. 1, Jan-Feb, (1977).
\nGuinchen, G., and Gai, V., “Controle du climate autour de 197 instruments de musique,” 7th Triennial Meeting,
\nICOM Committee for Conservation, Copenhagen, 84.17.19-84.17.25, (1984).
\nKafui, K.D., “Transient Heat and Moisture Transfer in Thin Silica Gel Beds,” Transactions of the ASME, Vol. 116,
\npp. 946-953, Nov., (1994).
\nPolanyi, M., “Theory of Adsorption of Gases. A General Survey and some Additional Remarks,” Trans. Far. Soc.,
\nVol. 28, 316-333, (1932).
\nfile:\/\/\/C|\/Documents%20and%20Settings\/Owner\/My%20Documents\/websites\/museumClimateControls\/SGelCases.htm (17 of 18)12\/11\/2007 11:43:48 AM
\nAn Evaluation of Silica Gel for Humidity Control in Display Cases
\nSchweizer, F., “Stabilization of RH in Exhibition Cases: An Experimental Approach,” 7th Triennial Meeting, ICOM
\nCommittee for Conservation, Copenhagen, 84.17.50-84.17.53, (1984).
\nStolow, Nathan, “The Microclimate: A Localized Solution,” Museum News, Vol. 56(2): pp. 52-63, (1977).
\nThomson, G., “Stabilization of RH in Exhibition Cases: Hygrometric Half-time,” Studies in Conservation, Vol. 22,
\npp. 85-102, (1977).
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