Demystifying Silica Gel for Effective Microclimates January 22, 2019 Rachael Perkins Arenstein A.M

1/22/2019

DEMYSTIFYING SILICA GEL FOR EFFECTIVE MICROCLIMATES JANUARY 22, 2019 RACHAEL PERKINS ARENSTEIN A.M. ART CONSERVATION

Did you know that the internal surface area of a teaspoon of silica gel beads is equivalent to that of a football field? This amazing property makes silica gel a relatively low-cost method for protecting collections on display and in storage from inappropriate or fluctuating environments. But not all silica gel is created equal. • Are you curious as to why silica from preservation vendors is more expensive than other desiccants or gel from commercial vendors? • Have you found loose silica beads or cartridges and pondered why it no longer seems to be “working”? • Interested in guidance on how much to use for your application? Whether it comes in sachets, cartridges or loose beads, the manner in which it is installed and cared for will greatly affect its efficacy. This webinar will focus on understanding the different types of gel, why it matters which you use, and how to calculate appropriate amounts for use in a vitrine or storage cabinet. The basics of appropriate preservation environments, creating a safe microclimate and how to monitor the environment within will be touched on.

N.B. This version of the presentation contains additional information that was not included in the live webinar on January 22, 2019.

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 1 1/22/2019

Access the presentation at: https://www.connectingtocollections.org/effective-microclimates/

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 1 1/22/2019

OUTLINE • OVERVIEW OF AGENTS OF DETERIORATION, TYPES OF DAMAGE & THE NEED FOR ENVIRONMENTAL CONTROL • WHAT IS A MICROCLIMATE? • HOW DO WE CONTROL MICROCLIMATES? • WHAT IS SILICA GEL? • USING SILICA GEL EFFECTIVELY • CALCULATING AMOUNTS • PLACEMENT • MONITORING • RECONDITIONING • STORING

I’m pleased to be able to talk to you today about silica gel and how we can use it to meet some of the environmental needs of our collections. While serving as a member of the Connecting to Collections Care Professional Development Working Group I volunteered to present this topic as I was spending a lot of time working with my museum clients correcting misunderstandings about silica gel and how it can be effectively used.

For those of you who already have a sense of collection care basics, I hope that you’ll stick with me while I first review some introductory material on deterioration and environmental control. This foundation is important in understanding our environmental needs and whether they can be fulfilled by using silica gel in a microclimate. Then we’ll get into the nitty gritty of silica gel types and formats and how to use it properly in common museum applications.

I hope that after this presentation, even if you feel that using some of the equations isn’t your thing, that you will be able to ask good questions of a preservation vendor in choosing a gel for your particular application.

POLL #1: Are you using silica gel… • In storage cabinets or microclimates • In exhibition vitrines

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 2 1/22/2019

• Both • I’m not using silica gel but want to know more

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 2 1/22/2019

REVIEW OF ENVIRONMENTAL ISSUES

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 3 1/22/2019

Types of Deterioration

Chemical Mechanical

Biological

http://protectheritage.com/Lisbon2011/wp-content/uploads/2011/12/Linden-frinal-corrected-reduced-file-size.pdf

Our goal, as collection care and preservation professionals is to extend the life of the material in our care. I find the way that the Image Permanence Institute discusses deterioration to be useful in understanding what we are concerned about. IPI uses three broad categories: • Chemical • Mechanical • And Biological

Chemical deterioration refers to the reactions occurring within the object. This occurs naturally and inevitably as an item ages but can be sped by environmental factors such as temperature. Examples include change of color in photographs, fading of dyes, degrading plastics or foams, weakened textile fibers, yellowed and embrittled books and paper and corrosion of metal artifacts. Mechanical refers to physical damage such as cracking, warping, delamination, slumping. Cracking like that seen on this wood sculpture is an example of mechanical damage caused by fluctuating relative humidity. Biological damage means i.e. mold/fungi or insects and vertebrate pests. These biological factors that act on art can be heavily influenced by the environment. Mold growth requires periods of sustained high humidity and humidity. Elevated

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 4 1/22/2019

temperatures will speed insect reproduction cycles and some insect pests are known indicators of high humidity as they feed on mold and fungi.

Images: Jeremy Linden “Preservation Metrics: A Preservation Analysis Tool for Risk Analysis in Preventive Conservation” http://protectheritage.com/Lisbon2011/wp- content/uploads/2011/12/Linden-frinal-corrected-reduced-file-size.pdf

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 4 1/22/2019

Agents of Deterioration

Environmental Agents • Incorrect relative humidity (RH) • Incorrect temperature • Light • Pollutants

https://www.canada.ca/en/conservation-institute/services/agents-deterioration.html

We often speak about the Agents of Deterioration, which are defined as primary threats to our collections. Probably the best description of these 10 general categories, is available on the Canadian Conservation Institute’s website. Our job is to determine how we can detect, block, report, and treat the damage these Agents cause.

Today we are going to focus on one of the Environmental Agents - Incorrect Relative Humidity or RH, as it is a major factor into all three forms of deterioration.

Reference: CCI website https://www.canada.ca/en/conservation-institute/services/agents- deterioration/humidity.html#tft2a

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 5 1/22/2019

Incorrect Relative Humidity 1. RH too high

2. RH too low

3. Fluctuating RH

4. RH above or below an object specific critical value

https://www.canada.ca/en/conservation-institute/services/agents-deterioration/humidity.html#tft2a

CCI breaks down Incorrect RH in four ways. https://www.canada.ca/en/conservation- institute/services/agents-deterioration/humidity.html#tft2a

1. RH too high – High relative humidity is responsible for corrosion of metals, mold growth and mechanical damage of hygroscopic organic artifacts. High RH is generally considered 65% and above. Mold germinates at 70% and above so most guidelines for upper RH limits top out around 60%.

2. RH too low – Organic materials will lose moisture to the ambient environment if RH is too low. Generally, 30% is considered the lower limit. Below that, artifacts may shrink becoming desiccated and embrittled..

3. Fluctuations in RH – This has been, for many institutions, the most problematic issue in environmental control. When RH fluctuates, the moisture content of the artifact will fluctuate as the piece seeks equilibrium with its environment. This results in expansion and contraction. For artifacts made of multiple materials that react differently to changes in RH, or artifacts that are constrained or under tension, the differing rates of equilibrium can cause cracking, splits and other forms of

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 6 1/22/2019

damage.

4. RH above or below an object specific critical value – There are some types of artifacts that have specific environmental needs based on their materials, composition or condition. For instance archaeological metals benefit from low RH while unstable glass requires a moderate 40-60% RH.

So our goal is to prevent extremes, and other times it is to provide a specific set of conditions.

Images: eClimateNotebook Graphs: Author’s and IPI’s https://www.imagepermanenceinstitute.org/resources/newsletter- archive/v20/ipi-research-equilibration

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 6 1/22/2019

http://www.conservation-us.org/docs/default-source/hhi/hhisummary.pdf?sfvrsn=2

And we know that we often have trouble doing it. The Heritage Health Index Report published in 2005 was the first comprehensive survey to assess the condition and preservation needs of U.S. collections. And the report documented the challenges that many institutions have in creating appropriate environments for long-term preservation of our nation’s cultural heritage. A quarter of institutions reported no environmental control for their collections. And clearly, even ones that did had seen damage.

So you are not alone if you are grappling with these issues.

Heritage Health Index Summary report - http://www.conservation-us.org/docs/default- source/hhi/hhisummary.pdf?sfvrsn=2

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 7 1/22/2019

The 70/50 Debate

https://www.academia.edu/1500445/2010._The_Plus_Minus_Dilemma_A_Way_Forward_in_Environmental_Guidelines

The Heritage Health Index Report resulted in a number of initiatives, among them the C2CCare platform we are on now, aimed at providing information to address preservation issues. And, along with that we also have seen a real shift over the past decade in recognizing that some of what people thought they needed to do, like providing strict environmental control at 70 degrees Fahrenheit and 50% RH plus or minus 5%, may not only be unachievable for some institutions, it may be unsustainable for most, and unnecessary for many.

So, that brings us back to our topic at hand

Images: https://www.academia.edu/1500445/2010._The_Plus_Minus_Dilemma_A_Way_Forward_i n_Environmental_Guidelines eCNB graph of flatline Temp and RH

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 8 1/22/2019

MICROCLIMATES

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 9 1/22/2019

What can we control?

https://www.canada.ca/en/conservation-institute/services/agents-deterioration/humidity.html

We know that we want to control RH, but we recognize for most of C2CCare’s core audience, controlling the entire building environment may be impossible. It may be easier to control one specific space, like this storage room on the right depicted in one of CCI’s case studies. But if recent research is showing that many collections can withstand a broader range of RH conditions than maybe we previously thought, it might be most efficient to focus on controlling the environment for our most valuable, important or sensitive items like you see here on the left.

Images: Canadian Conservation Institute website - https://www.canada.ca/en/conservation- institute/services/agents-deterioration/humidity.html

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 10 1/22/2019

What is a microclimate?

And the way we can do this is by creating microclimates

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 11 1/22/2019

What is a microclimate?

Images (L to R): https://www.canada.ca/en/conservation-institute/services/preventive-conservation/guidelines-collections/archaeological-collections.html http://vikingmetal.com/case-studies/custom-cabinets-art-museum/ https://www.glasbau-hahn.com/en/display-cases/display-case-technique/climate-control-technology/climate-control-systems/

For our purposes, whether in storage or on display, this can mean using a container, cabinet or vitrine, or other form of containment, to provide an environment for a select grouping of artifacts that is different from the surrounding ambient environment. Microclimates can be used to protect our collections from incorrect relative humidity as well as a whole range of other agents of deterioration such as pollutants, physical damage, light, water, theft and more.

But it is important to recognize that elements like a tight seal on a cabinet and vitrine, that makes a microclimate good for controlling RH, can also have adverse effects if pollutants are trapped inside.

Images (L to R): https://www.canada.ca/en/conservation-institute/services/preventive- conservation/guidelines-collections/archaeological-collections.html http://vikingmetal.com/case-studies/custom-cabinets-art-museum/ https://www.glasbau-hahn.com/en/display-cases/display-case-technique/climate-control- technology/climate-control-systems/

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 12 1/22/2019

CONTROLLING MICROCLIMATES

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 13 1/22/2019

Active versus Passive

Images (L to R): 1. Active control unit on the back of a vitrine - https://www.collectioncare.org/museum-microclimates-line-course 2. Xergy’s Xumidor unit controlling the environment in a vitrine 3. Silica gel chamber under a vitrine displaying archaeological metals 4. Oxygen absorber and desiccant RP System sachets used by conservator Ellen Carrlee for post-treatment control https://ellencarrlee.wordpress.com/2013/06/13/shipwreck-doll/

There are two ways of controlling microclimates:

Active environmental control includes HVAC systems and, on a smaller scale, systems like those sold by Jerry Shiner of Keepsafe (far left) or the new Xumidor proton exchange units (second from left), where you are using an electronically powered mechanism to change the environment inside your microclimate.

In contrast, passive control used in the vitrine and storage bag seen here on the right side, doesn’t need electricity. Instead it relies on having some sort of hydrophilic buffering material that will create a reservoir to either absorb or give off moisture within the enclosure. The process works by diffusion and so it takes some time.

POLL #3: Are you using active and/or passive measures for controlling RH? • Active • Passive • Both • Not sure

Images (L to R):

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 14 1/22/2019

1. Active control unit on the back of a vitrine - https://www.collectioncare.org/museum-microclimates-line-course 2. Xergy’s Xumidor unit controlling the environment in a vitrine 3. Silica gel chamber under a vitrine displaying archaeological metals 4. Oxygen absorber and desiccant RP System sachets used by conservator Ellen Carrlee for post-treatment control https://ellencarrlee.wordpress.com/2013/06/13/shipwreck-doll/

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 14 1/22/2019

Active versus Passive: How do I choose?

• Humidity level and range desired • Ambient conditions • Enclosure size • Enclosure materials • Leakage rate • Exhibition duration • Composition and size of the artifact • Resources

In choosing between an active or passive system you will want to consider: • Humidity level and range desired in the microclimate and how different that is from the ambient conditions • The size of the enclosure, materials and construction, as that will determine its leakage rate – meaning how fast the air inside and outside will be exchanged • How long you need the microclimate to work – is this a short term exhibit lasting three months or basically a permanent display or storage solution • The size and composition of the collection items will determine how well they buffer themselves. • And of course practical considerations like cost, staffing, expertise.

Resources: Shiner - http://www.keepsafe.ca/?page_id=50

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 15 1/22/2019

Active versus Passive: How do I choose?

Active Passive Large and/or leaky enclosures Small and/or tight enclosures Substantial difference in the Moderate or dry environment desired climate from the Buffering versus changing the ambient environment Long-term need Short or long-term needs Power source is available No power source Higher level of resources Limited resources

Both types of systems require time for setup and maintenance. Generally active systems require a lot of investment in the setup but should be easy to maintain. Whereas passive systems may be easier to set up but need regular ongoing maintenance.

Today we’re going to be focusing on passive control but there are some other good webinars on the C2CCare site that delve into appropriate environmental conditions, HVAC systems and other active measures. Check out: • Alice Carver-Kubik’s webinar - Storage Environments: the Big Picture https://www.connectingtocollections.org/storage-environments/ • Mibach and Conrad’s webinar - When Less is All You Got! Budget-conscious solutions to protect collections on display and in storage. https://www.connectingtocollections.org/when-less-is-all-you-got/

Reference: Shiner - https://renwickfas.com/knowledge/active-and-passive-microclimate-generation/

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 16 1/22/2019

OVERVIEW OF SORBENTS

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 17 1/22/2019

Materials for Passive Control

There are multiple sorbents that can be used for passive environmental control. And they are used in all sorts of industries. You’ve seen those little packets with your new pair of shoes that say DO NOT EAT! And the packets in with electronics, pill bottles or food that needs to stay dry. Let’s quickly review what some of these materials are before we focus in on silica gel.

POLL #4: Are you using any other passive method for controlling RH on display? • Salt solutions • Activated clays • Molecular sieves • Nope, only silica gel • I’m not currently suing any method of passive control

Screen shots: Protective Packaging Corp - https://www.protectivepackaging.net/clay-desiccant Brownell Limited - https://brownell.co.uk/products/desiccants/molecular-sieves.html University Products - https://www.universityproducts.com/ Gaylord Archival - https://www.gaylord.com/

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 18 1/22/2019

Materials for Passive Control - Salts

https://www.conservationphysics.org/mm/shiner/shiner.pdf

The earliest attempt to control the environment in display cases appears to date to the 1930s where a British patent was awarded to a specially designed museum case with built in trays to hold salt solutions. Different salts, when made into a slushy, saturated solution will maintain a specific RH in a contained environment. So for instance, at room temperature around 20 degrees Celsius or 68 degrees Fahrenheit, Lithium Chloride will hold at 11%, Magnesium Chloride at 33% and Sodium Chloride at 75% RH. You can easily find tables of these measurements online. This was a low tech method for buffering an environment, but over time the salts can creep over the sides of the containers and make a mess. Plus conservators are generally leery of adding salt in proximity to our collections.

Images: Trends in microclimate control of museum display cases Jerry Shiner - https://www.conservationphysics.org/mm/shiner/shiner.pdf Wikipedia - https://en.wikipedia.org/wiki/Lithium_chloride#/media/File:Lithium_chloride.jpg Equilibrium ReIative Humidity Saturated SaIt Solutions - https://www.omega.com/techref/pdf/z103.pdf

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 19 1/22/2019

Materials for Passive Control - Salts

That is why we stay away from desiccants salts like these calcium chloride bags sold by ULINE. But we still use this technique to create environments for checking calibration of equipment like dataloggers as you see on the right. In this outer box there is an inner box with the salt solution covered by a breathable fabric.

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 20 1/22/2019

Materials for Passive Control – Activated clays

Activated clays like montmorillonite and bentonite are naturally occurring minerals that you’ll see sold. Because they aren’t a synthesized products they are generally cheaper to buy. These clays are chemically inert and non-toxic making them safe to use and dispose of. The clays work well as a desiccant, with the ability to absorb around 25% percent of their weight in water without swelling, but they are not as good as a buffering material so they can be less useful for some of our museum applications. They aren’t as efficient as silica gel so you may need to have space for more clay packs than you would need in silica gel. So ultimately, while it is an acceptable solution, it probably isn’t an efficient use of your preservation funds.

There are other sorbents like molecular sieve zeolites, and activated alumina, but these are not commonly used for general museum applications, so I don’t want to spent time on them today. It is good to understand that there are other materials out there that work as sorbents, but you would have to have a very specific application to make their properties or their costs worthwhile.

Images: Uline screen shot http://www.absorbwell.com/desiccant/clay-desiccant/100g-clay-desiccant-bag.html Protective Packaging Corp - https://www.protectivepackaging.net/clay-desiccant

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 21 1/22/2019

SILICA GEL - THE NITTY GRITTY

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 22 1/22/2019

Materials for Passive Control – Silica Gel

https://commons.wikimedia.org/wiki/File:SilicaGel.jpg.jpghttps://commons.wikimedia.org/wiki/File:Silica_gel_pb092529.jpg

So that brings us to silica gel. But before I start talking about details, I want to mention two important publications that form the best resources for this topic. They are also listed at the end of this presentation and in the accompanying handout and they are available free online. 1. The first is the Canadian Conservation Institute’s 2018 update of Technical Bulletin 33 – Silica Gel: Passive Control of Relative Humidity by Jean Tetreault and Paul Begin. 2. And the second is the 2002 paper Demystifying Silica Gel by Steve Weintraub, of Art Preservation Services. I will be pulling liberally from these two publications with the permission of the authors.

Silica gel is a chemically inert, non-toxic material composed of amorphous silicon dioxide. It is a synthetic of form of silica and is produced in either granular or beaded form. Beaded silica gel has a higher mechanical strength than the granular form so it generates less dust and is therefore our preferred form. Silica gel was patented in 1919 for use in the adsorption of vapors and gases in gas mask canisters during World War I (Weintraub, 2002, p.4) and by the 1970’s was a common solution for controlling RH in museum displays (Shiner, 2007, p.269)

Images: https://commons.wikimedia.org/wiki/File:Silica_gel_pb092529.jpg

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 23 1/22/2019

https://pub.cci-icc.gc.ca/resources-ressources/publications/category-categorie- eng.aspx?id=18&thispubid=539

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 23 1/22/2019

Materials for Passive Control – Silica Gel

Silica gel beads have a vast network of internal pores creating a lot of surface area. Weintraub describes that the surface area of one teaspoon of silica gel beads, if spread out, would be approximately the size of a football field! The gel is so successful as a desiccant because it adsorbs moisture into its pores – up to 40% of its weight in some cases.

Images: Teaspoon of silica gel - http://www.fuji- silysia.co.jp/english/product/micronized_silica/index.html Football field - https://en.wikipedia.org/wiki/Gridiron_football#/media/File:AmFBfield.svg

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 24 1/22/2019

Silica Gel Format & Types

Silica gel comes in various forms. It is cheapest to buy as loose beads but it is often more convenient to contain it in a cartridge or sachet for ease of handling and use. The sachets are generally made from some sort of permeable fabric like Tyvek or Reemay. Cartridges should be designed to maximize surface areas. The gel can also be made up into sheet form. This is best used for specific applications like microclimates for paintings rather than general control.

You’ll also hear various product names, gel types and grades mentioned such as Rhapid Gel, PROSorb, Art-Sorb, Regular Density (RD), High Performance (HP), Type A, B, C. To understand what these terms mean and why they are important we have to take a closer look at how silica gel works as a sorbent.

Images: SmallCorp - http://www.smallcorp.com/wp-content/uploads/2018/01/SmallCorp-Silica- Gel-Options.pdf University Products - https://www.universityproducts.com/dri-can-reusable-desiccating- canister.html Talas - https://www.talasonline.com/Art-Sorb-Beads Talas - https://www.talasonline.com/Art-Sorb Talas - https://www.talasonline.com/Rhapid-Pack-Silica-Gel

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 25 1/22/2019

Buffering

L: Weintraub 2002, p. 2 R: https://www.imagepermanenceinstitute.org/resources/newsletter-archive/v20/ipi-research-equilibration

By using silica gel in a reasonably tight microclimate we can smooth out the peaks and valleys of our ambient environment so that our collections don’t feel such abrupt shifts either daily or seasonally. Most sorbents are used as desiccants. Think back to the examples of the food and pharmaceuticals where we see those little packets used. In museums, we sometimes use silica gel as a desiccant to reduce RH for instance to store metals at a lower RH than the other artifacts in storage require. But we mostly use it as buffering agent and that has some important implications for how we select and use silica gel.

To reiterate, using silica gel or any sorbent without a relatively well sealed microclimate is of limited value because the gel will eventually equilibrate to the ambient environment.

Ask POLL #2: If you use silica gel in your institution are you using it to… • Desiccate • Buffer • Raise RH

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 26 1/22/2019

• All of the above

Images: Figure 1: Weintraub 2002, p. 2 - https://docs.wixstatic.com/ugd/cb7feb_1cb7f80734314e3ebbfa3fdd0de514e8.pdf IPI’s https://www.imagepermanenceinstitute.org/resources/newsletter- archive/v20/ipi-research-equilibration

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 26 1/22/2019

Equilibrium Moisture Content

L: Weintraub, 2002, p. 2 R: http://www.connectingtocollections.org/wp-content/uploads/2017/11/StorageEnvironments_ACK_C2C_December2017.pdf

Organic materials are hygroscopic – they absorb and release water depending on the RH of the surrounding air. This process will continue until the interior moisture content of the artifact reaches equilibrium with its environment. This is the Equilibrium Moisture Content (EMC). IPI’s research has demonstrated that moisture equilibration (in contrast to thermal equilibration) is relatively slow. Depending on the nature of the material, size and surface area, it may be days or weeks until an artifact equilibrates to a change in RH. Placing artifacts into a microenvironment like a box or cabinet will slow this process even further.

In Weintraub’s chart on the left you can see that silica gel (both the regular density and Artsorb) is more responsive in terms of buffering capacity when RH changes than organic materials such as wood, paper or wool. This means that the gel will buffer the environment in an enclosure faster than the artifact. Surface area within the buffering material is what determines the speed of response. So, in this case, the silica beads have more surface area than, for instance, the text block of a book. Generally, we don’t want our collections to buffer themselves.

Images:

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 27 1/22/2019

Figure 1: Weintraub 2002, p. 2 - https://docs.wixstatic.com/ugd/cb7feb_1cb7f80734314e3ebbfa3fdd0de514e8.pdf Alice Carver-Kubik, Image Permanence Institute, Storage Environments: The Big Picture, Connecting To Collections Care webinar http://www.connectingtocollections.org/wp- content/uploads/2017/11/StorageEnvironments_ACK_C2C_December2017.pdf

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 27 1/22/2019

Buffering Capacity – M Value

• M value - the amount of water (in grams) that is gained or lost by 1 kilogram of silica gel for each 1% change in RH.

• Variables affecting M include: • The point along the EMC/RH isotherm at which it is measured. • The magnitude of the RH range used to determine M. • Whether it is measured along the adsorption or desorption isotherm. • Hysteresis

Weintraub, 2002 p.4

“The capacity of different types of gel to buffer is affected by factors such as capillary pore size or the inclusion of hygroscopic salts, resulting in a wide range of performance. Therefore, it is important to compare the buffering capacity of different types of silica gels to determine which silica gel has the best performance for a specific application.” (Weintraub, 2002 p.5) This variable has become known as the “specific moisture reservoir” and is described with the variable M.

But M varies due to several factors including: • The point along the EMC/RH isotherm at which it is measured. • The magnitude of the RH range used to determine M. • Whether it is measured along the adsorption or desorption isotherm. • Hysteresis

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 28 1/22/2019

What the @$*#% is Hysteresis?!

When we measure the M value of a gel, the adsorption curve as RH rises is not the same as the desorption curve as RH goes back down. The lag, which causes a flattening out of the curve in the upper RH range is hysteresis.

Image: CCI Technical Bulletin 33 - https://www.canada.ca/en/conservation- institute/services/conservation-preservation-publications/technical-bulletins/silica-gel- relative-humidity.html#a11b

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 29 1/22/2019

Buffering Capacity – MH Value • Since the M value can change based on whether the gel is adsorbing or desorbing moisture (i.e.hysteresis), we can use MH value to take this into account.

• MH value - the average amount of water (in grams) that is gained or lost by 1 kilogram of silica gel for each 1% change in RH.

• The (H) designates that hysteresis is accounted for within the specified RH range.

• A higher MH value indicates a higher buffering capacity.

Weintraub, 2002 p.4

To simplify, if we just want to use silica gel as a desiccant then we can use the M value. BUT, we often want to use silica gel as a buffer. And if we are both adsorbing and desorbing, then we need to take hysteresis issues into account.

So instead we refer to MH which is a better measure of a gel’s buffering performance. MH is calculated by repeatedly cycling silica gel between adsorption and desorption within a specific RH range until a constant value is measured and hysteresis is taken into account,

MH reflects actual buffering performance. Ultimately, the higher the MH better it buffers.

(Weintraub, 2002 p.5 and personal communication)

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 30 1/22/2019

Buffering Capacity of Different Gels •A-type - Good moisture adsorption capacity between 0-50% RH. Above 50% RH, the capacity to adsorb moisture diminishes. •B-type and C-type - Low moisture adsorption capacities below 70% RH and are not appropriate for humidity buffering applications below 70% RH. •High-Performance Silica Gels – Special gels with good buffering characteristics between 0-70% RH. Examples include RHapid Gel, ArtSorb and PROSorb.

The silica gel industry divides silica gel into three “standard” types, based on the pore size of the gel: A-type, B-type, and C-type. • A-type Silica Gel has good moisture adsorption capacity between 0-50% RH. Above 50% RH, the capacity to adsorb moisture diminishes. For this reason, Type-A gel is an effective humidity buffering material within the range of 0-55% RH when used in sufficient quantity. It is not recommended for humidity buffering applications above 60% RH. Type A is also sometimes called regular density or RD gel. • B-type and C-type have low moisture adsorption capacities below 70% RH and are not appropriate for humidity buffering applications below 70% RH. This means we normally don’t use these kinds of gels in museums. But you might find them in your kitty litter! • High-Performance (HP) Silica Gels refer to special types of silica gel that have good humidity buffering characteristics between 0-70% RH. RHapid Gel, Art Sorb and PROSorb fall within this category.

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 31 1/22/2019

Buffering Capacity – HP versus RD

Jean Tétreault and Paul Bégin, 2018 https://www.canada.ca/en/conservation-institute/services/conservation-preservation- publications/technical-bulletins/silica-gel-relative-humidity.html#a11b

In the CCI Technical Bulletin you can see their MH calculations for several types of sorbents. So, what does this mean? Does it mean that Rhapid Gel, PROSorb and Art-Sorb are better

gels? Not necessarily! It depends on your application. For these three gels the MH value is highest in the mid-range and above. So these high performance gels will perform better than others if you are trying to buffer or maintain environments between 50-60% RH. If you need to buffer at a moderate RH then it will be worth spending the money on one of these products because they will perform better. In the case of high performance gels, Rhapid Gel and PROSorb will perform similarly as they have a fairly linear M values through

the normal range of museum environments. With ArtSorb you can see that its MH value falls off in the lower humidity range.

Regular density gel and the Orange indicating gel which is a RD gel with a colorant, have

much lower MH values in the mid to upper RH range, but they perform as well or better at lower relative humidities. So, if you are using your gel as a desiccant you can save money by buying a regular density Type A gel from SmallCorp or another preservation vendor, or the new Arten Gel from Art Preservation Services, rather than a high performance gel.

And you can see that the gels are better buffers than the desiccant clay and zeolites.

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 32 1/22/2019

Buffering Capacity – MH for A-Type Silica Gel

RH Below 40-50% 45-55% 50-60% Above Range 45% 60%

MH 6.0→5.0 3.5 2.5 1.5 1.0

But if you are looking to maintain a humidity below 40% then a A-type gel is comparable to the high performance gels. That 45-50% RH range is really where you see the split between the RD and High Performance (HP) gels. You need to compensate by using a lot more A get to get the equivalent buffering at high RH.

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 33 1/22/2019

CALCULATING AMOUNTS

So now that you hopefully know a bit more about how it works in theory, let’s discuss practical issues.

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 34 1/22/2019

How Much Do I Use?

The question of how much gel to use is one that I am frequently asked by my clients. There is no simple way to answer this question although there are basic concepts to be aware of:

If you are simply trying to desiccate the environment in a microenvironment it may simply want to put in as much dry gel as you can fit. But if you are trying to buffer around a specific RH or there are other more specific considerations, then knowing a bit more will

help you get a better result. And key to this is the MH value (buffering capacity) that we discussed before.

If Gel #1 has half the MH value of Gel #2, then you would need to double the amount of Gel #1 to achieve the same buffering capacity as Gel #2. Therefore, if one company claims that you need less gel than another company, they must be able to support the claim that their

gel has a higher MH value. If it doesn’t, then their claim can’t be correct. So if a manufacturer/vendor states that you only need X quantity of gel and gives no information on how that information number is derived then you have to be wary.

This is why it is critical to know the MH value for any gel because without it you can’t do a comparison. Not all vendors publish the MH value – and not all may even know it. If a vendor gives a really high MH value for a Type A gel that doesn’t seem right, you may want to call to ask for the technical information or testing that supports that number. Refer back

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 35 1/22/2019

to the CCI Technical Bulletin for their independent tests of some of the common products.

If you have selected a gel then, how do you calculate how much you actually need for a particular vitrine or cabinet?

Images: Talas - https://www.talasonline.com/Art-Sorb Long Life for Art - http://www.cwaller.de/english.htm?eprosorb.htm~information

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 35 1/22/2019

How Much Do I Use?

Some sites have seemingly user friendly tools such as calculators that allow you to plug in your case volume. These should be considered gel specific but, they also must be examined carefully. Because if the vendor doesn’t explain the variables that they are using, a mistake in their equation will translate to a mistake in the results and a problem in your vitrine.

In order to accurately calculate amounts of gel for an application you want to take into account other important information: • How leaky is your case or cabinet? • What is the ambient condition versus the condition you want to maintain?

• Do you know an accurate MH value for the gel you plan on using? • How tight do you need your control to be? • Is this a temporary exhibit or do you need to control conditions for a longer period?

The ArtSorb calculator on their website does not take these variables into account. The calculator from SmallCorp allows you to put in your variables, but it is still limited in its

efficacy and/or accuracy because they don’t give information on the performance (MH value) of their gel. So therefore you will not know the relative reliability of their calculated amount versus another gel. If this calculator is based on a non-linear type of silica gel, like a Type A gel, you would need very different amounts if you are calculating for a low RH

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 36 1/22/2019

environment versus a high RH environment.

Images: SmallCorp - https://www.smallcorp.com/silica-gel-calculator/ ArtSorb - http://www.artsorbonline.com/

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 36 1/22/2019

How Much Do I Use?

Jean Tétreault and Paul Bégin, 2018 https://www.canada.ca/en/conservation-institute/services/conservation-preservation- publications/technical-bulletins/silica-gel-relative-humidity.html#a11b

What lies behind those calculators is this general formula (found in the CCI Technical Bulletin and in Weintraub’s article). And, the best way to get an accurate result is to do the calculations yourself so you know the variables are accurate for your circumstances and data. Let’s walk through this using some explanatory information from Weintraub’s article.

Q = is the amount of silica gel in kilograms that we are calculating to find. Ceq = Concentration of water vapor at saturation. [At 22.7º C (73ºF), a cubic meter of air holds 20 grams of water vapor at saturation] D = The differential between the external RH and RH within the exhibit case. • e.g. For a permanent, maintenance-free case where the internal RH range is 45-55% and the exterior range is approximately 30%-70% RH, D is the difference between the RH midpoint within the case (50%) and the lowest or highest external RH values, 30%, or 70%. In either case, D = 20% or 0.2. • In another scenario, if the external RH is approximately 35%-65% and the environment in the vitrine is generally 45%-55% then D is the difference between the lowest internal and external RH values, 45% and 35%, or between the highest internal and external RH values, 55% and 65%. In either case, D = 10% or 0.1. V = The volume of the case in cubic meters. N = The number of air exchanges per day. When this data is unknown you can use a value of 1 to assume a typical moderately sealed case based on Gary Thomson’s, 1977

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 37 1/22/2019

publication. t = The maximum number of days that the exhibit case should remain within an acceptable range of RH. So you could use 90 days for a typical temporary exhibition or more for a permanent exhibit that you would like to be low maintenance.

MH = The moisture buffering capacity of silica gel within the specific RH range of use, taking hysteresis into account. This is variable discussed earlier in the presentation. F = The acceptable maximum range of RH fluctuation within the exhibit case. For example, for sensitive organic materials like ivory perhaps you’d want 45-55% RH.

Image & Resources: CCI Technical Bulletin 33 - Jean Tétreault and Paul Bégin, 2018 https://www.canada.ca/en/conservation-institute/services/conservation-preservation- publications/technical-bulletins/silica-gel-relative-humidity.html#a11b

Weintraub - https://docs.wixstatic.com/ugd/cb7feb_1cb7f80734314e3ebbfa3fdd0de514e8.pdf

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 37 1/22/2019

How Much Do I Use?...and how much will it cost?

To determine how much gel of different types to use, we can use this calculation to compare products. Here is a brief explanation of the variables used: Temp (Ceq) = 20 C is used here for general room temperature D = 0.1 for the differential between the external RH and RH within the exhibit case. V = For our imaginary case we will use 1 cubic meter. N = 1 based on based on Thomson’s value for a typical moderately sealed case t = 90 days as changing out gel more frequently is, realistically, a challenge for most institutions based on work load.

MH = This is the critical value you will change each time you do the calculation if you want to compare different gels F = We will use 20 as the value here meaning 50%+/- 10% change is acceptable in the vitrine.

If you use the formula and calculate the amounts using different types of gels that are sold, you quickly get a sense of the difference in amounts that may be required to achieve the same buffering or desiccating effect.

• If we are plug in a typical MH value of 1.9 for a regular density Type A gel then we get a quantity required of 4.7 kg for our imaginary vitrine to hold at 50% +/- 10% RH.

• If we plug in a MH value of 5.4 for ProSorb we would only need 1.7 kg for the same case in the same conditions.

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 38 1/22/2019

So what this means, if you want to use Type A gel for a higher RH environment it is fine, but you would need more than twice as much as you would in ProSorb.

For a low RH case, the MH value is different for the Type A gel as it’s absorption/desorption curve is non-linear (remember slide 29!). So for the same 1 cubic meter case between 20- 30% RH:

• If we plug in a MH value of 5.5 for the Type A gel we would need 1.6 kg • If we plug in a MH value of 4.4 for ProSorb we would need 2.0 kg gel

Next, if you want to calculate the actual purchase cost of gel to get equivalent performance, then you must take the unit cost per kilogram, and multiply it by the relative equivalent performance number. More details on these calculations and the relative cost for equivalent performance are given in the accompanying handout Comparing Performance and Cost of “Museum-grade” silica gels from the webinar page on the Connecting to Collections Care website.

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 38 1/22/2019

How Much Do I Use?

I find it helpful to use an Excel spreadsheet to ensure I don’t make mistakes in my math. The other common mistake I often see is inconsistency in using SI or Metric measurements. You may need to translate your measurements from one unit to another. Make sure you are consistent!

Another tip: If you are at an institution that has a stock of unlabeled product, sorting it out by type based on packaging may help you determine what kind you have.

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 39 1/22/2019

USING SILICA GEL

So now that you hopefully know a bit more about how it works in theory, let’s discuss practical issues.

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 40 1/22/2019

Common Mistakes

There are a couple of mistakes that I see frequently in my work. One is simply not using enough gel for the volume of air in the vitrine. On the right here you see a large case with four cassettes of gel. There was absolutely no desiccating effect going on here for these sensitive maritime iron artifacts. The case on the left had a lot more gel for a much smaller volume of air and was doing a good job maintaining the desired environment.

The other mistake that I see a lot is not taking into account the D value in the equation – meaning the difference between the ambient conditions and what you want to achieve in the microclimate if you are trying to change the environment rather than merely buffer it. If you have an ambient RH in your gallery of 45% RH and you want the case to be under 30% RH for your metals. You don’t want to put in 30% gel. You would want to put in dry gel, meaning gel that is under 10%, to pull the RH down far enough. With the same logic, if it is winter and you have a low ambient RH around 35% and your sensitive ivories need 50%. You don’t want to put in 50% gel. You need to pull it up with 60%. These numbers can be worked into that big equation to help guide quantities.

Images: Author

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 41 1/22/2019

Accessing Sorbents

Exhibit Conservation Guidelines CD-ROM, 1999 National Park Service

When using a sorbent on exhibit, the vitrine design will heavily influence the ease or difficulty in installing and maintaining the environment. The National Park Service Exhibit Conservation Guidelines CD-ROM nicely diagrammed a variety of ways to incorporate a silica gel chamber into a case. Design #1 requires access through the display chamber which is the least desirable style as you would have to deinstall the artifacts to maintain the gel. Most experienced exhibit fabricators have worked these ideas into their standard case designs.

Images: Exhibit Conservation Guidelines CD-ROM, 1999 National Park Service, Toby Raphael and Kevin Brookes

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 42 1/22/2019

Air Circulation

Exhibit Conservation Guidelines CD-ROM, 1999 National Park Service

Your case design must also provide for adequate air circulation between the environmental chamber and the vitrine where the artifacts are placed. This is often done either by having a gap around the display deck or perforating the deck. You want the air exchange rate between the sorbent and the artifacts to be higher than that of the interior of the case and the exterior ambient conditions. Otherwise you won’t see much benefit.

Images: Exhibit Conservation Guidelines CD-ROM, 1999 National Park Service, Toby Raphael and Kevin Brookes

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 43 1/22/2019

Case Construction

Here are a couple of examples of how that might look in an actual display case. The door is open to the bottom of the case you see the open bottom of the vitrine exposing the environmental chamber. This would be an example of the NPS guidelines Design #2 with a removable (and nicely gasketed access panel). But notice the small size of the chamber relative to the footprint of the deck. And the chamber to the deck with a narrow chimney further choking off airflow.

Images: Author

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 44 1/22/2019

Circulation

This was a test on a vitrine where we weren’t seeing any effects from the gel in the environmental chamber at the bottom of the vitrine. Not only was the chamber small for the large size of the case, it had a narrow chimney connecting it to the display and there was clearly no circulation. We placed silica gel into the vitrine to demonstrate to the fabricators that it was the case design that was a problem, not the gel itself. The curator made up a nifty sign to explain what was going on during the testing.

Images: Courtesy of Susan Jones

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 45 1/22/2019

Silica in the vitrine

Exhibit Conservation Guidelines CD-ROM, 1999 National Park Service

There are more attractive ways to hide silica gel in the vitrine bonnet with the artifacts. You can purchase or construct risers or blocks like this one on the left called the RHiser from SmallCorp, Gaylord or other preservation vendors that hide silica gel cartridges or sachets underneath or behind.

Images: SmallCorp - https://www.smallcorp.com/risers/ Gaylord - https://www.gaylord.com/Exhibit-%26-Display/Display-Accessories/Risers%2C- Raised-Decking-%26-Shelves/Humidity-Controlled-Linen-Wrapped-Riser/p/HYB02600

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 46 1/22/2019

Installing

Here is another example. This vitrine was used to display archaeological iron and copper alloy artifacts and the lender wanted the case held under 20% RH for the six months of the exhibit. With limited staffing the host institution wanted it to be as low maintenance as possible. The case had a large footprint so we put in as much dry gel as we could fit. The bottom was lined with Marvelseal to improve the seal of the chamber.

Images: Author

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 47 1/22/2019

Installing

The artifacts were placed on slant boards over large cutouts in the deck that allowed the air to circulate into the bonnet. The seal of the bonnet was improved with a silicone gasket. The arrow shows where an Arten thermohygrometer was placed to monitor the conditions. The case maintained a RH under 20% for the six months of the exhibit.

Images: Author

For more on gaskets review: “A set of conservation guidelines for exhibitions”, 2000. Toby Raphael and Martin Burke. Objects Specialty Group Postprints, Volume Seven, 2000. American Institute for Conservation. http://resources.conservation-us.org/wp-content/uploads/sites/8/2015/02/osg007-02.pdf

“Airtight, humidity stabilized display cases: The practical design and fabrication of sealed exhibit cases”, 1991. Toby Raphael. Objects Specialty Group Postprints, Volume One, 1991. American Institute for Conservation. http://resources.conservation-us.org/wp- content/uploads/sites/8/2015/03/osg001-08.pdf

And look for products that have tested well on AIC’s wiki Oddy Testing Materials Database

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 48 1/22/2019

pages http://www.conservation-wiki.com/wiki/Oddy_Test_Results:_Case_Construction_Materials

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 48 1/22/2019

Silica Gel In Action

Here you can see the dramatic effect on the environment on a vitrine after silica gel was installed. Over a couple of hours the RH dropped from 47% to under 20%. The fact that the RH began a steady creep upwards suggested to me that the case’s leakage rate wasn’t quite what they claimed it to be.

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 49 1/22/2019

Monitoring – Indicating Gels

Low RH High RH

How do you know when it is time to change out and recondition your gel? Well, there are a few types of silica called “indicating gels” that use pH indicators, iron salts or heavy metals to create a change in color as the gels absorb moisture. There are a few common types these days: • Orange to green • Orange to white • And blue to pink (30% range is where it really starts to change color) We see less of the blue to pink kind these days as the cobalt dichloride used to give the color was classified as a carcinogen in the European Union.

You don’t need a ton of indicating gel. It is generally more expensive than uncolored beads but you can scatter some in with large amounts of non-indicating gel as a quick visual check like you see here in this jar from Talas.

This is a good opportunity for a brief digression…

Images: Silica gel desiccant bags - https://silicageldesiccantbags.wordpress.com/tag/silica-gel- orange-bead/ International Silica Gel Co. Ltd - http://www.hysilicagel.com/cgi/search-

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 50 1/22/2019

en.cgi?f=product_en_1_+company_en_1_&id=637285&t=product_en_1_ Adsorbents & Desiccants Corporation of America - https://www.adcoa.net/product- category/silica-gel/silica-gel-grade-42/ Talas - https://www.talasonline.com/Silica-Gel-Bulk-Pack

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 50 1/22/2019

Health & Safety

• Non-toxic and non-reactive under normal conditions. • Avoid contact with high heat or strong acids Hazardous decomposition products: • Health Effects Inhalation: Synthetic amorphous silica gel has little adverse effect on lungs when exposure is kept below the permitted limits but dust can aggravate medical conditions e.g. asthma. • Eye Contact: Dust may cause discomfort and mild irritation. • Skin Contact: Dust may have a drying effect on the skin • Carcinogenicity: Amorphous silica is not carcinogenic to humans but additives such as color indicators like blue cobalt dichloride are.

To talk about health and safety issues. Silica gel is generally safe for use. As with any material, it is recommended that you request and then read the Safety Data Sheet from the supplier. When I am working with large amounts of silica gel I recommend using a dust mask and gloves, particularly if you are handling loose beads where there is sometimes crushed silica dust.

Health & Safety info e.g. SDS can be found at: http://cameo.mfa.org/wiki/Silica_gel White silica gel - https://beta-

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 51 1/22/2019

static.fishersci.com/content/dam/fishersci/en_US/documents/programs/education/regulator y-documents/sds/chemicals/chemicals-s/S25519.pdf Orange silica gel - https://www.sorbentsystems.com/pdf/orangesilicagelmsds.pdf Blue indicating silica gel - http://www.ebd.csic.es/lie/PDF/SILICA%20GEL.pdf

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 51 1/22/2019

Monitoring – Humidity Indicator Cards

http://resources.conservation-us.org/wp-content/uploads/sites/8/2015/03/osg021-06.pdf

I’m not a fan of loose gel as it is a pain to work with. It can be fine though in small quantities. In storage microclimates like this one, a simple humidity indicator card may be sufficient to allow someone to occasionally check when gel needs to be dried out.

The image on the left is pulled from a good article by Dana Senge that discusses various issues in creating microclimates for storage.

Images: Dana Senge, 2014. “Testing and Implementation of Microclimate Storage Containers”, Objects Specialty Group Postprints, Volume Twenty-One, AIC. http://resources.conservation-us.org/wp-content/uploads/sites/8/2015/03/osg021- 06.pdf OTE - http://www.preservationcare.com/shop/humidity-indicator-cards

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 52 1/22/2019

Monitoring – Thermohygrometers & Dataloggers

If you are using cassettes or sachets you are better off with either a monitor that you can place in the vitrine and see, like these simple thermohygrometers on the left, or a datalogger that you don’t have to physically connect to like these reasonably priced Bluetooth loggers from Onset or Lascar. You can download the data from the loggers to a phone or iPad to see what the conditions are inside the case. While they are more expensive than the hygrometers, you can more easily track what is going on in between spot checks and share the data with lenders to document your conditions.

Images: Arten - https://www.carrmclean.ca/arten-mechanical-thermo-hygrometer.html Talas thermohygrometer - https://www.talasonline.com/archival-storage/environmental- controls Onset Bluetooth logger - https://www.onsetcomp.com/products/dataloggers/mx1101?creative=178262173812&keyword=&matchtype=&network=s&device=c& gclid=Cj0KCQiA- JXiBRCpARIsAGqF8wVYz1SHhmaLPSIKM9JmTFNI4gKmfFaob5ZoTtD9Ztk1FfjeUksTZ3AaAjac EALw_wcB Lascar Bluetooth logger - https://www.lascarelectronics.com/easylog-data-logger-el-bt-2/

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 53 1/22/2019

Monitoring

Here you can see the environmental conditions in a vitrine that we wanted to bring down to under 30% RH in a gallery with an ambient RH of around 48% RH. There was a dramatic drop when we added the silica gel but the differential in conditions was great enough and the case not so air tight, that within a week we were back over 30% RH and we had to replace the gel again. The next time the gel was drier but it still was only two months before the interior conditions were crept up over 30%.

Ask for POLL #5: Have you ever tested the air tightness of your vitrines? Or do you have the info from your fabricator on the air exchange rate? • Yes • No

Images: Author - Environmental data

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 54 1/22/2019

Drying Gel

Drying gel out so it can be used again as a desiccant is easy although potentially time consuming. Since most of us don’t have a fancy drying oven like on the left here, you can place your silica gel beads, cassettes or sachets in a regular household oven or even toaster for gentle heating. Most gels can be heated to about 250Fahrenheit (120 Celsius). Indicating gels may require slightly lower temperatures and ARTSorb should not be heated above 150Fahrenheit (65 Celsius) to prevent cracking of the beads. Most manufacturers and distributors do NOT recommend using a microwave for drying. However, even if the silica gel bead can withstand a reactivation temperature around 250F there may be other factors relating to the casing or enclosure (i.e. cassette, sachet, pouch) that calls for a lower temperature. The cassettes may have adhesive joins that can soften at high heats. And the sachets or pouches are often made of synthetic material like Reemay (polyester spunbonded fabric) or Tyvek (spunbonded polyethylene) that can melt in high heat. So generally I advise heating no higher than 200F to ensure that you don’t damage either the gel or the casing. If you are using a oven, use the convection setting as it moves the air around evenly. If using a toaster you may want to use a the lowest heat setting as it the smaller oven can create a closer more intense heat that might be too much for your gel or packaging.

When you dry gel generally the first bit of water comes off quickly but getting it really dry will take a lot more time. Generally anything under 10% RH would be considered dry, from

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 55 1/22/2019

a practical point of view. The amount of time the drying process will take will depend on the ambient humidity and what RH the silica has become equilibrated to. But the process can take hours or even days. This should be planned into exhibition schedules to ensure there is enough time between venues or installations.

There is no limit to the number of times you can recondition silica gel beads.

Images: https://wandamech.en.made-in-china.com/product/nsrxWHkAshcz/China-Drying-Oven-for- Pharmaceutical-Silica-Gel-Drying-Oven.html Author - Silica gel sachets in the oven at 120 degrees F

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 55 1/22/2019

Reconditioning Gel

Adding moisture to recondition silica gel to higher relative humidities to use as a buffering agent in a microclimate is a bit more complicated. The image on the left is a shot of the gel conditioning chamber created by Art Preservation Services. What you can’t see in the picture is humidity control equipment and fans that generate the target RH condition within the chamber. This setup allows for uniform conditioning of silica gel sachets because of the large surface area of silica gel that is exposed to precise RH conditioned air within the sealed chamber.

A couple of other companies sell their own reconditioning units. In the middle you see the large unit sold by SmallCorp. This system, which is designed to work with their sachets can condition up to 50kg of gel at a time. It costs about $2,700. Both Gaylord and SmallCorp sell smaller reconditioning units that range in price from $350 - $800 and can condition 3 kg of gel at a time.

These units may save some effort on manipulating the gel, but it is important to recognize the limitations. Uniform and precise conditioning cannot be achieved by exposing a mass of silica gel to a high but uncontrolled RH, where weight of added water is used to determine the RH level to which the silica gel has been

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 56 1/22/2019

equilibrated. The reason is that the moisture level is not uniform within the bulk gel, and the process is prone to error because it does not take hysteresis into account.

The CCI bulletin also shows a method for creating a simple reconditioning setup.

Images (left to right): Art Preservation Services conditioning system – courtesy of S. Weintraub SmallCorp large reconditioning unit - http://www.smallcorp.com/wp- content/uploads/2018/01/SmallCorp-Silica-Gel-Options.pdf Gaylord Archival® Silica Gel Reconditioning System Part #: 61-GBCONDSM - https://www.gaylord.com/Exhibit-%26-Display/Installation-Supplies/Gaylord- Archival%26%23174%3B-Silica-Gel-Reconditioning-System/p/61-GBCONDSM

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 56 1/22/2019

Reconditioning Tips • Spread the gel as thin as possible. • Use a fan to circulate air around the gel. • Periodically mix the gel layers to improve uniformity. • Allow time for moisture to equilibrate within and between the gel beads, especially if beads with different moisture contents are mixed together. • Do not add water directly to gel as it can cause cracking of the beads.

Weintraub, 2002, p. 12

The Demystifying Silica paper by Weintraub gives some calculations for other ways to recondition to a target RH as well as these generally useful reconditioning tips: • Spread the gel as thin as possible. • Use a fan to circulate air around the gel. • Periodically mix the gel layers to improve uniformity. • Allow time for moisture to equilibrate within and between the gel beads, especially if beads with different moisture contents are mixed together. • Do not add water directly to gel as it can cause cracking of the beads.

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 57 1/22/2019

Checking Dryness

If you are reconditioning or have gel lying around and you aren’t sure what it is equilibrated to, you can easily set up a simple microclimate to check what is going on.

On the left I’m using one of those Bluetooth loggers to see if I have dried out the sachets as low as I need. I removed the pouches from the oven, let them cool to room temperature an then put them into this polypropylene bin. The bin isn’t well sealed as the gel doesn’t remain in here for very long.

On the right, I drilled a hole in the Tupperware container to fit the Elsec 765 meter’s probe allowing me to check the RH level of the cartridges. This container is thicker plastic with a gasket and will hold conditions inside for longer.

Images: Author

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 58 1/22/2019

Checking Dryness

Here is the graph from the logger in the previous slide. #1 shows my first check of the gel (blue line). The heating I did brought it down from 45% to just under 20% but that wasn’t good enough. #2 was when the chamber was empty. The RH pops up as I took the gel out and put it back in the oven . #3 shows that after the next baking it was under 10% which was where I wanted it.

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 59 1/22/2019

Storing Gel

Once your gel is dry or conditioned to the RH you want, you can use a barrier film like Marvelseal to create bags or pouches to maintain it until it is needed. I found these resealable bags on Amazon. And on the right you can see ArtSorb cassettes stored in a similar pouch. Storing gel in regular plastic Ziploc style bags won’t keep them at the right RH for very long because, unlike barrier films like Marvelseal, Aclar or Escal, the bag is permeable.

Images: Author - Resealable barrier film bags for storing gel available from https://www.amazon.com/gp/product/B01IWRNSWO/ref=ppx_yo_dt_b_asin_title_o04__o 00_s00?ie=UTF8&psc=1

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 60 1/22/2019

SUMMING UP

There is a lot more to learn and I really encourage you to check out the CCI Technical Bulletin, read Weintraub’s article and look carefully at other vendor websites to delve into the data or notice the lack of it! It is relatively simple to use silica gel a buffering agent without knowing too much about the gel or how it works. But, the more you demand of the gel, that is, the more you need it to provide precise conditions, the more you need to know about the gel’s specific properties. I hope that this introduction will provide a bridge to the more technical data that is available.

Images: http://www.conservationsupportsystems.com/product/show/beaded-silica-gel-type- a/silica-gels

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 61 1/22/2019

Technical Resources

• Silica Gel: Passive Control of Relative Humidity – Technical Bulletin 33, Jean Tétreault and Paul Bégin, Canadian Conservation Institute https://www.canada.ca/en/conservation- institute/services/conservation-preservation-publications/technical- bulletins/silica-gel-relative-humidity.html • Demystifying Silica Gel, Steve Weintraub, Art Preservation Services https://docs.wixstatic.com/ugd/cb7feb_1cb7f80734314e3ebbfa3fdd0 de514e8.pdf

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 62 1/22/2019

Vendors

• Art Preservation Services https://www.apsnyc.com/ • Carr McLean (Canada) https://www.carrmclean.ca/ • Gaylord https://www.gaylord.com/ • Keepsafe Microclimate Systems http://www.keepsafe.ca/ • Long Life for Art (Europe) http://www.cwaller.de/english.htm • SmallCorp https://www.smallcorp.com/ • Talas https://www.talasonline.com/ • University Products https://www.universityproducts.com/

Inclusion or exclusion in this listing does not constitute an endorsement or approval of company or material

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 63 1/22/2019

Acknowledgements

• Steve Weintraub, Art Preservation Services • Jean Tétreault, Canadian Conservation Institute • Jerry Shiner, Keepsafe Microclimate Systems • Stephen Gorman, Museum of the Bible • FAIC and M. Susan Barger, Connecting to Collections Care

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 64 1/22/2019

QUESTIONS?

RACHAEL PERKINS ARENSTEIN A.M. ART CONSERVATION, LLC WWW.AMARTCONSERVATION.COM [email protected]

Connecting to Collections Care webinar, January 2019

Rachael Perkins Arenstein is a founding partner in A.M. Art Conservation, LLC a NY-based private practice with specializations in object conservation and preventive care. She has worked as a conservator at the Bible Lands Museum Jerusalem, Smithsonian National Museum of the American Indian, the Peabody Museum of Archaeology and Ethnology, the American Museum of Natural History, and institutional and private clients both nationally and internationally where she has been responsible for the preservation of collections, exhibit conservation and environmental monitoring. She also worked for Art Preservation Services in NYC conditioning silica gel and calibrating equipment for use in microclimates. Rachael’s degree in art conservation is from the University of London. She is a Professional Associate in the American Institute for Conservation (AIC) and active in several professional organizations including positions as the E-editor for the AIC and the Co-Chair of the Integrated Pest Management Working Group. She thinks tinkering with hygrometers is fun!

C2C Care Webinar, Demystifying Silica Gel for Effective Microclimates, Arenstein 65