Improved Understanding of 

Moisture Effects on Outdoor 

Wood–Adhesive Bondlines

Joseph E. Jakes

Nayomi Z. Plaza

Xavier Arzola Villegas

Charles R. Frihart

United States Department of Agriculture

Forest Service

Forest Products Laboratory

General Technical Report

FPL–GTR–246

May 2017

In cooperation with the 

United States Department of Transportation

Federal Highway Administration

A FHWA-NHCBP (National Historic Covered 

Bridge Preservation) publication
Abstract

The development of improved moisture-durable wood adhesives 

for outdoor applications, such as repairing historic covered bridges, 

is hindered by an incomplete mechanistic understanding of what 

makes a wood–adhesive bond moisture-durable. The wood–

adhesive bondline is extraordinarily difficult to study because of 

the chemical, structural, and mechanical complexities and 

variations of the wood substrate and the many potential 

interactions that can occur between the wood and adhesive to 

enhance or degrade the bond performance. Although the literature 

is filled with bonding information from the bulk level, sufficient 

experimental tools to study wood–adhesive bondlines and the 

effects of moisture on bondlines at the cellular and subcellular 

levels are lacking. Therefore, we recently developed tools to study 

bondlines at these size scales. We have used synchrotron-based 

micro x-ray fluorescence microscopy (μXFM) and micro x-ray 

computed tomography (μXCT) to map the penetration of adhesive 

into the wood anatomical structure. We also developed 

nanoindentation into a tool capable of assessing the effects of 

moisture and adhesive infiltration on wood cell wall properties. 

Using custom-built in situ humidity chambers with μXCT and 

small angle neutron scattering, we can now study the moistureinduced swelling in wood across the relevant length scales in 

bondlines that span from the millimeter to nanometer length scales. 

Finally, we developed a new approach to study moisture-induced 

swelling forces using wood slivers. This report gives an overview 

of these recently developed techniques. The insights these tools 

provide toward understanding moisture durability in wood 

adhesive bondlines are also discussed in this report.

Keywords: historic covered bridges, adhesives, moisture durability

May 2017

Jakes, Joseph E.; Plaza, Nayomi Z.; Arzola Villegas, Xavier; Frihart, 

Charles R. 2017. Improved understanding of moisture effects on outdoor 

wood–adhesive bondlines. FPL–GTR–246. Madison, WI: U.S. Department 

of Agriculture, Forest Service, Forest Products Laboratory. 7 p.

A limited number of free copies of this publication are available to the 

public from the Forest Products Laboratory, One Gifford Pinchot Drive, 

Madison, WI 53726-2398. This publication is also available online at 

www.fpl.fs.fed.us. Laboratory publications are sent to hundreds of libraries 

in the United States and elsewhere.

The Forest Products Laboratory is maintained in cooperation with the 

University of Wisconsin.

The use of trade or firm names in this publication is for reader information 

and does not imply endorsement by the United States Department of 

Agriculture (USDA) of any product or service.

Contents

Introduction.................................................................................... 1

Techniques..................................................................................... 2

Summary and Future Outlook ........................................................ 5

Acknowledgments.......................................................................... 6

References...................................................................................... 6

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This study is part of the Research, Technology, and Education 

portion of the National Historic Covered Bridge Preservation 

(NHCBP) Program administered by the Federal Highway 

Administration. The NHCBP program includes preservation, 

rehabilitation, and restoration of covered bridges that are listed 

or are eligible for listing on the National Register of Historic 

Places; research for better means of restoring and protecting 

these bridges; development of educational aids; and technology 

transfer to disseminate information on covered bridges in order 

to preserve the Nation’s cultural heritage.

This study is conducted under a joint agreement between the 

Federal Highway Administration–Turner Fairbank Highway 

Research Center and the Forest Service–Forest Products 

Laboratory.

Federal Highway Administration Program Manager—Sheila 

Rimal Duwadi, P.E.

Forest Products Laboratory Program Manager—

Michael A. Ritter, P.E.
Introduction

Repairing historic covered bridges may require bonding new 

wood members to existing wood members or replacing 

original large solid timbers with glulam beams, which are 

made by bonding small dimensional lumber together until 

the glulam beam is large enough to replace the original solid 

timber. Thus, adhesives with moisture durability are needed

to bond these materials. Although the roof structure keeps 

the heavy timber trusses sheltered and dry enough to survive 

more than 100 years, the components still experience 

exposure to water and fluctuating humidity. The effects of 

moisture play a critical role in the longevity of the adhesive 

bond’s service life. Moisture sorption causes dimensional 

changes in wood, and if wood–adhesive bondlines cannot 

accommodate the continuous swelling and shrinking caused 

by changes in moisture content, then stress can concentrate 

and bondline failure can occur. Although there are moisturedurable adhesives for making wood products, there are not 

suitable adhesives for repair of wood products. Epoxy 

adhesives are often used because they are the best that are

available with suitable cure, flow, and gap-filling properties, 

but they have limited moisture resistance in wood bonds

(Frihart 2006, AITC 1994). Most exterior-rated forest 

products use phenol formaldehyde (PF) adhesives because 

of their strength and moisture durability, but these adhesives 

cannot be used for repair of wood structures in the field. 

However, if we better understood why PF adhesives form a 

moisture-durable bond with wood, we could use this 

knowledge to accelerate the development of adhesives that 

could be used in the field for repairing historic covered

bridges.

The majority of research in wood adhesives has 

concentrated on bulk performance and failure analysis at the 

bulk level. However, failure begins at the molecular level 

and continues to propagate until it reaches the bulk level. To 

develop improved adhesives for repairing covered bridges, 

research is needed to better understand the effects of 

moisture on wood and the interactions between wood and 

adhesives across multiple length scales from molecular to 

bulk levels. To aid in this research, it is useful to identify the 

different regions of a typical wood adhesive bondline 

(Fig. 1). Bulk wood is defined as the wood far enough from 

the bondline that its properties are not affected by the 

adhesive’s presence. The bulk adhesive is the adhesive layer 

that separates the two wood adherends. The interface is the 

exposed surface of the wood adherend that the adhesive is 

applied to, which typically consists of cut wood cell wall

and lumen surfaces. The interphase contains the region of

wood modified by adhesive. During ambient humidity 

fluctuations, the moisture content of both the wood (Glass 

and Zelinka 2010) and adhesive (Wimmer and others 2013)

also fluctuate accordingly, with higher humidity leading to 

higher moisture contents. As the amount of absorbed water 

in the wood product changes with changing moisture 

content, dimensional changes can occur. However, 

differences in the magnitudes or rates of swelling and 

shrinking between the different regions in a bondline can 

result in strain mismatches and stress concentrations that 

serve as the locus for bondline failure. To produce moisturedurable bondlines, wood adhesives must be able to manage

differential swelling and shrinking to prevent failure. One 

hypothesis is that durable structural wood–adhesive 

bondlines are formed when the adhesive penetrates into the 

nearby wood forming a wood–adhesive interphase region 

that minimizes any stress concentrations that form between 

the bulk adhesive and wood adherends (Frihart 2009).

At present, the most common moisture-durable wood 

adhesives typically need to be applied in a mill setting in a 

controlled environment with carefully prepared wood 

surfaces, but these conditions are not amendable to making

repairs to standing wood structures in the field, such as 

covered bridges. Of the durable wood adhesives available, 

PF is one of the best moisture-durable wood adhesives. 

Although the general consensus is that its durability arises 

from the penetration of PF into the wood cellular structure 

Improved Understanding of Moisture Effects 

on Outdoor Wood–Adhesive Bondlines

Joseph E. Jakes, Research Materials Engineer

USDA Forest Service, Forest Products Laboratory, Madison, Wisconsin

Nayomi Z. Plaza, Student Training General Engineer

Xavier Arzola Villegas, PhD Graduate Student

USDA Forest Service, Forest Products Laboratory, Madison, Wisconsin

Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin

Charles R. Frihart, Research Chemist

USDA Forest Service, Forest Products Laboratory, Madison, Wisconsin
General Technical Report FPL–GTR–246

2

and cell walls to create an interphase region (Kamke and

Lee 2007, Frihart 2009, Stamm and Seborg 1936), the 

underlying molecular- and nano-scale mechanisms that give 

the PF–wood interphase its favorable properties are still 

debated. An improved mechanistic understanding of how PF 

and other moisture-durable adhesives produce moisturedurable bondlines would accelerate the development of new 

types of wood adhesive systems that could be used in the 

repair of wood structures in the field. However, the 

multiscale structural, chemical, and mechanical 

complexities of the bondline in combination with its small 

size make wood–adhesive bondlines extraordinarily difficult

to study, especially measuring basic parameters such as

adhesive penetration, swelling strains, swelling stresses, and

mechanical properties of individual components in the 

bondline.

In this report, an overview is given of recently developed

research techniques being used to study wood and wood–

adhesive bondlines to improve our mechanistic 

understanding of how to create moisture-durable wood–

adhesive bondlines. To demonstrate their utility, the 

techniques will be applied in the study of PF bondlines.

How synchrotron-based micro x-ray fluorescence 

microscopy (μXFM) can be used to map the adhesive 

infiltration into wood cell walls will be shown. Conjointly,

nanoindentation can be used to assess moisture-dependent 

mechanical properties of the same μXFM-mapped cell walls 

to directly correlate adhesive infiltration of wood cell walls 

in the interphase to their moisture-dependent mechanical

properties. It will also be demonstrated that micro x-ray 

computed tomography (μXCT) experiments can be used to 

map adhesive penetration into the wood cellular structure 

and visualize moisture-induced swelling in wood–adhesive 

bondlines. We also developed small angle neutron scattering 

(SANS) techniques to better study how the nanostructures

of wood cell walls (1- to 100-nm-length scale) swell with 

moisture uptake. Finally, a technique will be presented to 

study moisture sorption and swelling forces in wood slivers, 

which are composed of a few wood cells that could be 

excised from wood–adhesive bondlines to assess moisture 

sorption and swelling stresses in bondline interphases. By 

combining information from these techniques using the 

same sample source (wood, adhesive, and bonding 

conditions), we will learn more about the adhesive 

interactions with wood that limit the formation and 

propagation of nano- and microscale failures. Additionally, 

in the future, the developed research techniques will be used

to study and develop other adhesives for making them more 

moisture-durable and suitable for the in-field repair of 

outdoor wood structures, such as covered bridges.

Techniques

Mapping Adhesive Infiltration into Wood Cell 

Walls

Techniques are needed to accurately map and quantify the 

adhesive infiltration into wood cell walls to better correlate

the amount of adhesive cell wall infiltration with changes in 

cell wall properties and adhesive performance. We 

developed synchrotron-based μXFM into a tool to 

accurately map adhesive infiltration into wood cell walls 

with submicron spatial resolution (Jakes and others 2013, 

2015). In μXFM, an x-ray beam is focused to a submicron 

spot size and then raster scanned over a specimen while an

energy dispersive detector system quantifies photons

fluoresced from any element inside the x-ray beam 

interaction volume. Because the energy of fluorescence 

photons is characteristic for each element, the elemental 

content and amount of element present at the sample spot 

can be determined. We used μXFM to map a brominelabeled phenol formaldehyde adhesive (BrPF) at beamline 

2-ID-E at the Advanced Photon Source at Argonne National 

Laboratory (Argonne, Illinois). The Br maps, which are in 

direct proportion to the amount of adhesive infiltration into 

the cell wall, are shown in Figure 2. Wood–adhesive 

bondlines were made with southern yellow pine (Pinus 

taeda) latewood and five BrPFs with different molecular 

weight (MW) distributions. From each bondline, 2-μm-thick

cross sections were cut and imaged. An increasing MW of 

BrPF is shown from left to right in Figure 2. With μXFM,

we were able to observe that the higher the MW the less the 

BrPF infiltrated into the cell walls and the less the adhesive 

penetrated the wood adherend. In some sections, especially 

at lower MW, individual lumina were filled with adhesives 

far from the bondline. Subsequently, using μXCT, we will 

show that the adhesive reached these lumina via ray cells 

that connected to the bondline in a plane outside of the 

section cut for μXFM. The red lines in Figure 2 correspond 

to the rows of cells tested with nanoindentation (next 

section).

Figure 1—Optical microscopy image representative of a 

typical wood–adhesive bondline cross section. The image 

is approximately 0.15 mm per side. The bondline was 

made using a phenol resorcinol formaldehyde adhesive to 

bond ponderosa pine (Pinus ponderosa). The top and 

bottom adherends were earlywood and latewood, 

respectively.
Improved Understanding of Moisture Effects on Outdoor Wood–Adhesive Bondlines

3

Moisture-Dependent Mechanical Properties of 

Wood Cell Walls

Nanoindentation is a technique capable of measuring 

hardness and elastic modulus of micron-sized volumes of 

materials. More than a decade ago, this technique began to 

be used as a tool to assess the hardness and elastic modulus 

of wood cell walls and adhesive layers in situ in wood–

adhesive bondlines (Gindl and Gupta 2002, Gindl and 

others 2004b). In contrast to conventional macroscopic 

hardness measurements, such as the Janka hardness test 

which assesses resistance to deformation in the cellular 

structure of wood, nanoindentation probes the properties of 

individual wood cell walls and adhesive layers. We used

nanoindentation in experiments on the rows of wood cell

walls highlighted by red lines in Figure 2 to quantify the 

effects of adhesive infiltration on the moisture-dependent 

mechanical cell wall properties (Jakes and others 2015). 

Figure 3 shows an atomic force microscopy image of 

representative nanoindents placed in the S2 cell wall layer. 

The moisture-dependent cell wall properties were studied by 

controlling the relative humidity (RH) inside the

nanoindenter enclosure, and experiments were performed 

after conditioning the bondlines in dry air and 78% RH. For 

the control cell walls without BrPF infiltration, the hardness 

and elastic modulus decreased by about 50% and 25%, 

respectively, when conditioned at 78% RH compared with

dry air (Jakes and others 2015). The relative effect of BrPF 

infiltration on cell wall mechanical properties was found to 

depend on the humidity level. In dry conditions, there was 

minimal effect of BrPF infiltration on mechanical 

properties. However, at high humidity, the higher the 

amount of BrPF infiltration the less the cell wall mechanical 

properties decreased compared with those for the dry 

conditions. There was also an effect of BrPF MW on the 

hygromechanical properties. Interestingly, for the same 

amount of BrPF infiltration, the lower MW BrPF adhesives 

were found to be more effective at minimizing mechanical 

softening at higher humidity by as much as a third (Jakes 

and others 2015). This greater effectiveness of lower MW 

BrPF probably resulted from their ability to more intimately 

associate with water sorption sites in the wood polymers.

These results suggest that when designing a moisturedurable wood adhesive, just getting adhesive infiltration into 

the wood cell walls may not be sufficient. There is also 

probably specific molecular- and nano-scale interactions

between the adhesive and wood that must occur inside the 

cell wall to create a moisture-durable bondline.

Adhesive Penetration and Moisture-Induced 

Swelling in the Cellular Structure

Micro x-ray computed tomography (μXCT) is a 

nondestructive technique useful for creating threedimensional models with micron spatial resolution of 

complex solid structures. It has been found to be ideally 

suited for the study of wood–adhesive bondlines (Paris and 

others 2013, Kamke and others 2014). In μXCT, a series of 

two-dimensional x-ray images are taken of an object 

through a wide range of rotation angles and then used to 

computationally reconstruct a three-dimensional image of 

the object. We recently expanded the capabilities of μXCT 

by developing an in situ RH chamber (Fig. 4a) for beamline 

2-BM-B at the Advanced Photon Source at Argonne 

National Laboratory to visualize the three-dimensional 

structure of wood–adhesive bondlines and swelling caused 

by changes in ambient RH. By tagging adhesives with an 

electron-rich atom, such as Br, the x-ray attenuation of the 

adhesive increases and, using segmentation software, the 

adhesive can be visualized separately from the wood cell 

Figure 2—X-ray fluorescence microscopy (XFM) images of 

bromine-labeled phenol formaldehyde adhesive (BrPF) 

infiltration into wood cell walls. The Br intensity was 

normalized to the intensity value of the bulk BrPF in the 

bondline or BrPF-filled lumina. Red lines indicate rows of 

cell walls tested with nanoindentation. (MW, molecular 

weight.)

Figure 3—Atomic force microscopy image of nanoindents 

in the S2 layer of wood cell walls in an adhesive bondline 

(BrPF, bromine-labeled phenol formaldehyde adhesive).
General Technical Report FPL–GTR–246

4

walls. This was demonstrated using a specimen cut from a 

BrPF bondline used in the μXFM study, and the resulting 

reconstructions are shown in Figure 4b,c. In Figure 4c,

adhesive can clearly be observed flowing down rays and 

into longitudinal tracheid lumina far from the bondline, 

which explains how the adhesive arrived in the tracheid

lumina far from the bondline in some of the bondlines in

Figure 2. Figure 4d,e shows an adhesive bondline 

conditioned at 100% and 0% RH, respectively. Roughly the 

same cells are shown in Figure 4d,e, and the black box 

around Figure 4e shows how much the wood structure 

shrank during drying. Swelling strains of approximately 

10% can be observed. Although a more detailed analysis is

needed to quantify strains, it is interesting that in the 

direction parallel to the bondline, the swelling strain is not 

noticeably different close to and far from this particular

bondline. With the in situ RH chamber, μXCT has now been 

developed into a tool that can be used to study how 

adhesives accommodate, or conversely fail because of, 

moisture-induced swelling at the bondline. The swelling in 

the adhesive, interphase, and bulk wood can now be studied 

separately, and this information can be used to develop 

improved wood adhesives.

Moisture-Induced Swelling in Wood 

Nanostructure

Moisture-induced swelling in wood is caused by the 

absorption of water molecules in the polymeric structure of 

wood cell walls. However, it is not well understood how this 

molecular-scale absorption relates to the swelling observed 

at larger length scales, such as the cell wall nanostructure or 

the cellular length scales observed with μXCT (Fig. 4).

Therefore, recognizing the ability of SANS to probe 

nanostructure (1–100 nm) in organic materials such as

wood, we developed SANS into a tool to study the swelling 

in the cell wall nanostructure (Plaza and others 2016). We 

designed and built an in situ humidity chamber (Fig. 5a) for 

the Bio-SANS instrument at the High Flux Isotope Reactor 

and EQ-SANS instrument at the Spallation Neutron Source, 

which are both located at Oak Ridge National Laboratory 

(Oak Ridge, Tennessee). In particular, we found SANS 

useful for quantifying swelling in the nanostructure of 

cellulose microfibrils in wood cell walls. The microfibril, 

which is about 15 to 20 nm across, consists of an organized 

array of cellulose elementary fibrils, each of which is about 

3 nm across (Fig. 5b). During moisture sorption, water is 

absorbed by the less ordered regions between the 

elementary fibrils and can cause the spacing between the 

elementary fibrils to increase by as much as 1 nm (Fig. 5c). 

Figure 4—(a) Custom-built in situ relative humidity chamber for beamline 2 BM-B at the Advanced Photon Source at 

Argonne National Laboratory used to study moisture-induced swelling of wood structure; (b) three-dimensional 

reconstruction of wood adhesive bondline; (c) the same reconstruction as (b) but with the wood cellular structure 

computationally removed to reveal the adhesive; (d) tomographic slice of wood adhesive bondline conditioned at 100% 

RH; and (e) tomographic slice of wood adhesive bondline conditioned in dry air.
Improved Understanding of Moisture Effects on Outdoor Wood–Adhesive Bondlines

5

Through simple calculations, we found that in the secondary 

cell wall, more than half of the moisture swelling can be 

attributed to the swelling of cellulose microfibrils. This 

provides new insight to researchers trying to develop 

adhesives to minimize swelling in interphase wood cell 

walls. The adhesives must be able to infiltrate and modify 

the cellulose microfibrils to minimize swelling in wood cell 

walls.

Measuring Swelling Forces and Moisture 

Sorption of Interphase Wood Cells

We have found that a wood sliver, consisting of a few intact 

wood cells (Fig. 6a, b) twists as function of humidity (Plaza 

and others 2013, Jakes and others 2014). The twist is caused 

by the moisture swelling of amorphous wood polymers (for 

example, lignin, hemicelluloses, and amorphous cellulose)

in the secondary cell wall and the constraint along the 

longitudinal direction of the cellulose microfibrils, which 

are helically wound through the wood cell wall. By 

attaching a twist indicator to a sliver (Fig. 6c) and 

conditioning it at different humidity levels, we measured the 

sliver twist as a function of RH and found it to be directly 

proportional to the wood moisture content. Furthermore, by 

tethering the sliver with the twist indicator to a thin tungsten 

wire (Fig. 6d), we also showed how to assess the specific 

torque generated during moisture sorption. The sliver torque 

should be directly proportional to the swelling forces 

generated by water sorption in the amorphous wood 

polymers inside the wood cell walls, which are probably the 

same swelling forces causing bondline failures. We were

originally motivated to study wood slivers because they 

could be extracted from the interphase region of a wood–

adhesive bondline. Our developed sliver characterization 

techniques can now be used to gain new information about

how adhesive infiltration into wood cell walls modifies

swelling forces and water sorption in these cells as a 

function of humidity.

Summary and Future Outlook

We have developed innovative research tools that are

capable of providing new information about wood–adhesive 

bondlines at the cellular and subcellular levels and the 

effects of moisture on the bondlines. These tools should

accelerate researchers’ efforts to develop a mechanistic 

understanding of moisture-durable wood–adhesive 

bondlines and the development of wood adhesives for 

outdoor use for which moisture is an issue, such as repairing

historic covered bridges. Some of the techniques have 

already been applied to study the effects of moisture in 

wood and wood–PF bondlines. The combined results from 

these techniques are providing researchers with new 

insights. For instance, using μXFM and nanoindentation, we 

found that for a given level of cell wall infiltration, a lower 

MW BrPF was more effective than a higher MW BrPF at 

preventing mechanical softening at high RH. Also, using 

SANS, we found that more than half of the moistureinduced swelling in a secondary cell wall originates in the 

Figure 5—(a) Custom-built in situ relative humidity (RH) chamber for the Bio-SANS instrument at the High Flux Isotope 

Reactor at Oak Ridge National Laboratory (Oak Ridge, Tennessee). The humidity chamber can also be used with the EQSANS instrument at the Spallation Neutron Source at Oak Ridge National Laboratory. The humidity chamber and SANS 

were used to study moisture-induced swelling in the cellulose microfibril nanostructure, which is illustrated in (b) and (c). 

The elementary fibrils are semicrystalline cellulose that are not permeable to water. The less ordered regions are areas 

occupied by amorphous cellulose that can absorb water.
General Technical Report FPL–GTR–246

6

cellulose microfibrils. Combining these results leads us to 

propose that a lower MW BrPF may be better at infiltrating 

and modifying the cellulose microfibrils than a higher MW 

BrPF. If so, it may be that targeting and modifying the 

cellulose microfibrils is a key to dimensionally stabilizing 

wood cell walls during changes in RH.

The next step includes applying these techniques to study 

additional bondlines made with different types of wood 

adhesives that are known to be either moisture durable or 

not moisture durable to further understand which 

mechanisms are important. In particular, the adhesive 

system will probably need to minimize the moistureinduced swelling and shrinking of the wood near the 

bondline. An example of a system that greatly improves the 

moisture durability of a variety of adhesive systems, 

including epoxies, on a variety of wood species is the 

hydroxylated methylated resorcinol (HMR) primer (Vick 

and others 1995), but research at many institutes around the 

world have yet to understand why HMR works so well

(Gardner and others 2006; Hosen 2010; Kläusler and others

2014). Other systems for study include melamine

formaldehyde (Gindl and others 2004a) and one-component 

polyurethanes (Kläusler and others 2014), which are both 

widely used in Europe for exterior applications but not in 

the United States because they are often insufficiently 

moisture-durable to pass the required performance 

standards. After the primary mechanisms are better 

understood, improved adhesive systems can be designed for 

bonding new wood members to existing wood members in 

the repair of historic covered bridges.

Acknowledgments

Support is acknowledged from the Federal Highway 

Administration Cooperative Research Program for National 

Historic Covered Bridge Preservation. Nayomi Plaza

Rodriguez and Xavier Arzola Villegas acknowledge the 

Graduate Engineering Research Scholars program at UWMadison. Plaza Rodriguez acknowledges the 2012 National 

Science Foundation Graduate Research Fellowship Program

and USDA Forest Service Pathways program for support. 

Joseph E. Jakes acknowledges funding from 2011 USDA 

PECASE awards. The use of Oak Ridge National 

Laboratory High Flux Isotope Reactor and Spallation 

Neutron Source was supported by the Scientific User 

Facilities Division, Office of Basic Energy Sciences, U.S.

Department of Energy. The Bio-SANS beam line is 

supported by the Office of Biological and Environmental 

Research (FWP ERKP291), and its facilities are supported 

by the U.S. Department of Energy and managed by UTBattelle, LLC, under contract no. DE-AC05-00OR22725. 

The use of Advanced Photon Source facilities was 

supported by the U.S. Department of Energy, Basic Energy 

Sciences, Office of Science, under contract number W-31-

109-Eng-38. We acknowledge Tom Kuster, FPL Analytical 

Chemistry and Microscopy Laboratory, for the scanning 

electron microscopy images in Figure 6a and b and the 

machine shop at FPL for construction of the in situ relative 

humidity chambers used in the μXCT experiments (Fig. 4a) 

and SANS experiments (Fig. 5a). The covered bridge photo 

on the cover was taken by James P. Wacker of the Forest 

Products Laboratory. The bridge in the photo is the Flint 

Bridge in Orange County, Vermont.

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7

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