Visualization can greatly benefit understanding of concepts and processes, which in soil science and geology can be done using real-life snapshots of soils and sediments in lacquer peels and glue peels. While it may seem complicated, anyone can make such a soil peel for use in classrooms, public places, homes, and offices for teaching, outreach, decoration, and awareness. Technological development has considerably simplified the making of soil peels, but this methodological innovation has not been described in the literature. Here, we report on a thoroughly tested and simple method for taking peels of sandy soils using readily available tools and materials. Our method follows the main previously published steps of preparing a soil face, impregnating the soil face with a fixation agent in the field, extracting the resulting peel, and mounting it on a wooden panel. Yet instead of using lacquers and thinning agents, we use strong though flexible contact adhesive (glue), which has the major advantage that it no longer requires use and mixing of toxic chemicals in the field or reinforcement of the peel to prevent breaking. Moreover, the preservation potential is much higher than with the old method. This new twist to old methods makes creation of soil peels safer, simpler, and more successful, and thereby a true DIY (do-it-yourself) activity. The resulting increased accessibility of making soil and sediment peels can benefit research, teaching, and science communication and can thereby bring the value and beauty of the ground below our feet to students, schools, policy makers, and the general public.
Attention for soils is increasing around the world, in part due to strong
initiatives on soil health (Stott and Moebius-Clune, 2017; Schindelbeck et
al., 2008) and soil carbon (4 ‰, Minasny et al., 2017), explicit
articulation of how soils can help achieve the United Nations Sustainable
Development Goals (Keesstra et al., 2016; Bouma and Montanarella, 2016), and
the recent United Nations and IUSS declarations of the International Year of
Soils (FAO, 2015) and International Decade of Soils (IUSS, 2015),
respectively. The relevance of soils lies in the valuable beauty of soils:
their multidisciplinary functions and benefits (Brevik et al., 2015; Dominati
et al., 2010) and thereby their basis for life, in a world where soils are
under threat (Montanarella et al., 2016). Capturing this beauty in monoliths
or soil lacquer peels can bring soils
Soils and sediments can be fixated in two distinct ways: using peels and
monoliths. Both methods rely on impregnation of a soil face with a fixation
agent (such as lacquer, resin, or glue), and their final product is typically
mounted on a wall for study of undisturbed soil layers and characteristics,
or simply for decoration. Peels and monoliths are used to record and
illustrate a range of different features in soils, such as differences
between soil types, soil processes (e.g. weathering, gley, eluviation, and
illuviation of clay, iron, and organic acids; Fig. 1a), human impacts
(Fig. 1b), as well as biological activity such as plant rooting patterns,
burrowing of soil fauna, and bioturbation. Sedimentological and geological
processes can also be captured, such as cryoturbation, fluvial and aeolian
layering (Fig. 1c), frost wedges (Fig. 1d), and faults (Fig. 1e). And
finally, peels can show the splendid colours present in soils and sediments
(Fig. 1a–f). These natural snapshots of the subsurface are an effective way
to inspire people about soils (Megonigal et al., 2010) and geology, and are
used around the world by museums, universities, schools, and institutes
(Table 1) for teaching and outreach on the value of soils, the processes
occurring in soils, effects of management, and other factors. Interestingly,
these soil profiles are also used for testing knowledge of soils in job
interviews (Jacqueline Hannam, personal communication, 2018). Peels and monoliths allow
comparison of soils inside a classroom or museum environment without the need
for students or visitors to travel, allowing exposure to a variety of soils
in a short time and increasing accessibility of soil science to those with
disabilities that prevent them from observing soil in situ. Consequently,
soil science education at Wageningen University, the Netherlands, strongly
relies on a collection of
Lacquer peels showing
Example of 84 museums, universities, schools, and institutes with preserved soil profile collections (soil monoliths and/or lacquer peels) in 40 countries around the world.
Continued.
The main difference between making peels and monoliths is the location where the soil is impregnated with a fixing agent: a peel is impregnated in situ and extracted after drying, while a monolith is an undisturbed soil block that is extracted, transported, and then (repeatedly) impregnated in a laboratory (Van Baren and Sombroek, 1981). Monoliths can be created in any soil type, from sands to peats and heavy clays, but is rather time-consuming and requires specialized expertise in both the field and in the laboratory. Their creation and recent methodological developments are rather well described in scientific journals (e.g. Bouma, 1969; Haddad et al., 2009; Allaire and Bochove, 2006; Wessel et al., 2017; Wright, 1971; Donaldson and Beck, 1973; Barahona and Iriarte, 1999; Fitzpatrick et al., 2015), presentations (Fosberg, 2019), and reports (e.g. Van Baren and Bomer, 1979; Kiniry and Neitsch, 2019; Day, 1968; Schuurman, 1955), as well as illustrated in online videos and tutorials (e.g. University of Nebraska – Lincoln, 2016; Mueller, 2018). In contrast to soil monoliths, soil peels cannot be made from clay or peat soils since these are often too wet for impregnation in the field. Peels are therefore limited to relatively coarse sediments that retain less water (lower water holding capacity) and allow more rapid impregnation of fixation agents (because of their higher hydraulic conductivity), which is required in field situations. They thereby provide a rapid and accessible alternative to soil monoliths. The lack-film method for creating peels was first developed in the 1930s (Hähnel, 1962; Voigt, 1936; Jahn, 2006). Yet while the use of soil lacquer peels for scientific purposes has been recognized, e.g. to study sedimentological structures (Bijkerk et al., 2014; Van den Berg et al., 2007), for palaeo-geochemical analysis (Arnoldussen and van Os, 2015), or archaeological applications (Voigt and Gittins, 1977), the guidance available in the scientific literature is scattered, (out)dated, and/or incomplete. An English book that stands out is the comprehensive work by Bouma (1969) that details the history of soil and sediment peels as well as a range of fixation agents used to make these peels. Other published work includes a range of Dutch- and German-language papers, popular-scientific articles and reports (Vos et al., 2016; Huisman, 1980; TNO, 2010; Van Veen, 1985; Hähnel, 1961; Voigt, 1936), as well as a few older English-language articles (Voigt and Gittins, 1977; Van Baren and Bomer, 1979; Brown, 1963; Hähnel, 1962). These publications describe a range of materials used to make peels, most notably (nitrocellulose) lacquers, but also glue and resin.
The main steps of the published methods for making peels are straightforward: a soil face was prepared under an angle and then (repeatedly) impregnated with a fixation agent, with the peel extracted after drying and then mounted on a wooden board. The challenge of the published methods lies in the fixation agents used 50 years ago that required use of toxic solvents (e.g. acetone, toluene, xylene, thinner; Bouma, 1969) in the field to achieve the right viscosity, increasing the risk of harming people and the environment. The resulting peel was rather fragile, and hence reinforcement with cheesecloth or bandage was required to prevent rupture of the dried lacquer peel (Bouma, 1969). This fragility results in a lower preservation potential, which we have noted was especially challenging when the soil peels were frequently handled when used for teaching.
Here we describe and illustrate a simpler, safer, more durable, and thereby
more accessible approach to making soil lacquer peels, which relies on the
use of glue available at hardware stores. While still synthetic, this glue is
less harmful than the previous fixation agents, and its use straight out of
the can reduces the spilling risk associated with the mixing of chemicals in
the field. Finally, this method can be easily deployed by those who have
received no training. This new twist to an old method was developed by Gert
Peek, a soil science educator at Wageningen University who started teaching
at what was then the Laboratory for Soil Science and Geology at the
A range of materials is required to make a soil peel
that can typically be found in any hardware store. Table 2 lists all
materials required to prepare the soil face (a spade, pruning scissors or
garden shears, nail clippers, soil knife, ruler), to secure the soil (glue),
to extract the peel (wooden board, spade, soil knife, pruning scissors or
garden shears, garbage bag), to finish the lacquer peel (glue, notched
trowel, Stanley knife, nail clippers, scissors), and to mount it (hooks). In
terms of personal gear, garden gloves and clothes that can get dirty are
sufficient. Any size can be chosen for the final size of the soil peel, and
thus the size of the wooden board. Soil profiles at Wageningen University are
typically
Materials required and their purpose.
The fixation agent used to impregnate the soil face is a liquid contact adhesive based on neoprene rubber. Originally designed for shoe repairs that require two sides to be pressed together, this neoprene rubber contact adhesive works very well for making peels because it is flexible yet strong when dry. This flexibility is key for successful extraction of the peel from the soil face: glue that fully hardens when dry (like wood glue or glues used to impregnate monoliths in the lab) will break upon extraction and/or mounting of the peel. Another benefit of this glue is that it does not shrink when drying, unlike the lacquer used for instance by Hähnel (1962). In the Netherlands, neoprene rubber contact adhesive is sold as BisonKit Universal (Bolton Adhesives, Rotterdam, the Netherlands; Bison International, 2018b), which is internationally sold by the same manufacturer under the brand names Uhu Kontakt Kleber and Griffon Contact. The yellowish brown colour of this glue does not affect the colour of the final peel. Neoprene rubber contact adhesive is also known as polychloroprene glue, contact cement, or contact adhesive, and is elsewhere sold by manufacturers such as 3M, DAP Weldwood, Pliobond, and K-Flex-USA – check the suitability of these products in the field before purchasing large volumes. Because some of these brands still contain toluene, it is also advisable to request (material) safety data sheets (known as (M)SDS in the USA) to check for any required personal protective equipment.
The volume of glue (
Finding a good location can just be a matter of being outside a lot, knowing the surroundings, and scraping off the outer few centimetres of an exposed road cut to reveal the original soil underneath. In the old days (up to the 1990s) when workload at universities was still low, frequent and lengthy soil mapping field courses allowed for many opportunities to find beautiful soils and capture them in peels. Alternatively, with less time spent outside, good locations can also be found using digital maps that are often available online. Whether outside or behind a computer, four main factors determine the suitability of a location for making a soil peel: (1) soil texture, (2) groundwater depth, (3) a natural or man-made exposure, and (4) accessibility (Fig. 2a).
Main steps of making a soil peel.
First, regarding
Second,
Overview of locations suitable for making soil peels in the world
(sand content
Third, an
Locate the landowner and ask their permission. As many non-soil scientists do not know what a lacquer peel is, a simple explanation free of scientific jargon is to refer to it as a “soil painting” or “soil art”. Be honest about the use of glue, but also explain that you will clean everything up. Check whether the landowner would like to receive notice about the exact moment the fieldwork is planned – though as the process of making a soil peel is weather-dependent, this can often not be indicated much in advance, and acknowledge them in activities resulting from work on their land. Making soil peels can be an opportunity to involve land owners as an outreach activity, by having them on site, or sharing information (photo/video) about the process. In the dry summer of 2018, we created three soil peels on Wageningen University farmland with the pit excavated by Unifarm (farm services) – in return we made a fourth soil peel for Unifarm outreach activities.
In some climates, planning ahead for making lacquer peels can be challenging as this activity is rather weather dependent. Results are best when soils are dry, creating more intense colours and higher contrast of colours in the peel. In the Netherlands, our experience with the “soil profile weekends” taught that 2 weeks of dry weather in late spring or summer are sufficient to achieve good results. We have never had issues with soils that were too dry, and with the materials we use there is no need to spray the soil with water as suggested by Bouma (1969). While it is possible to make a peel when the soil is moist, the result is not as beautiful because of reduced appearance of for instance podzol fibres, or simply because the glue will not adhere to the sand. Note that while soil moisture contents may strongly vary in time, there may also be considerable differences within a soil profile. When sand may be already dry, horizons with more organic matter or clay can still be quite moist because of their strong effect on soil water retention (Rawls et al., 2003; Wösten et al., 1999). These within-profile differences may be exacerbated by impermeable layers: we once encountered major issues when extracting a peel from a podzol that had a perched water table due to an impermeable Bh horizon. While application of the glue (Sect. 2.4) was successful, the extracted peel showed that the glue had not adhered to the saturated E horizon above the Bh, while the C horizon below the impermeable layer was dry and adhered just fine. This peel was later restored in the lab (Sect. 2.6) using dried sand collected from the E horizon.
Dry weather is recommended both in the couple of weeks before making a peel
as well as during the 2 days in the field (Sect. 2.3–2.6), when air
temperature is also important. Follow the manufacturer's recommendations
regarding the temperature at which the glue can be used (e.g.
15–25
Use a spade to make a straight soil face at a
65
Cut away all roots protruding from the soil face using garden or nail clippers (for large and small roots, respectively) and remove any rocks or large rock fragments (Fig. 2c). Roots or rocks that stick out will retain glue and can thereby create glue-less pockets that will appear as holes in the finished lacquer peel. Cut the roots as close to the soil face as possible while avoiding any dislocation of sand grains. This can be a rather tedious process as the number of roots can be surprisingly high. Yet careful removal of roots and rock fragments will allow smoother impregnation of the soil face (Step 3), easier mounting of the peel on the wooden board (Step 5), and thus better final results.
Create a 5 cm ledge above the soil face (Fig. 2c), providing a place to pour the glue and preventing any soil material from above from falling onto the profile. If the top of the soil face is the same as the mineral soil surface, this ledge can be created by removing any litter and vegetation. If the top of the soil face starts mid-way a slope, this ledge can be made by simply cutting 5 cm into the soil.
Position a gutter underneath the soil face that can collect any excess glue (Fig. 2c). A piece of PVC pipe (diameter 10–15 cm) sliced in two and then capped on both ends can function as a good gutter, although a plastic bag may also do if positioned well. If the firmness of the soil profile allows, cut a 5 cm overhang below its bottom and locate the gutter underneath this overhang.
The total time required for field preparation strongly varies with the degree
of care taken when preparing the soil face – a general time estimate for
this step is
To allow for rapid application of the glue, open all
the cans of glue and place them within reach of the soil face – or close to
a helping hand who can give the cans to the person applying the glue. In
contrast to the previously discussed older methods that required on-site
mixing of glues or lacquers with thinning chemicals, glues used here are
ready-for-use and can thus be used straight out of the can. Application of
the glue on the soil face is easiest when using wide-mouth cans (
With the glue application done, the impregnation step of making the lacquer peel is finished. The neoprene rubber contact adhesive is so strong and yet flexible within the first days of application that it can easily hold the weight of a soil profile without tearing. As such, reinforcement of the peel with cheesecloth as directed by Bouma (1969) is not required. Collect any excess glue that is still liquid from the top ledge and the bottom gutter. Remove all trash and leave the site such that any visitors (people or animals) cannot harm themselves. Cover the impregnated soil face with a large (fisherman's) umbrella if there is a chance of light rain, and wait 20–24 h to let the glue dry (Fig. 2e). The exact drying time will depend on meteorological conditions (air temperature, relative humidity, and wind) and exposure of the profile. It may be that the profile is dry and ready for extraction after less than 20–24 h. Testing of potentially reduced drying times in different conditions is advisable in cases where time is tight and weather conditions are advantageous.
Extraction of the peel from the soil face involves the
repositioning of a lot of loose sand from behind the peel to the sides. To
facilitate this sand removal, make sure that the soil face on either side of
the impregnated section is flush with the lacquer peel for a width of
Everything is now in place to start digging
out the peel from above using the serrated edge of a (soil) knife. Starting
at the top ledge, use the (soil) knife like a saw to make a cut 5–10 cm
behind the glued soil face across the entire length of the peel (Fig. 2f,
Video S1). The knife cuts fine roots; use garden clippers to cut off larger
roots
Extraction of the soil peel is best done with two people, and can be done from the top (as outlined here) or from below (as outlined by Bouma, 1969). To extract the peel from the top, one person cuts away the soil and moves loose sand away from behind the profile and works their way down the profile. Once the top of the lacquer peel has been freed, a second person then presses a wooden board against the soil face that supports the top of the profile against the board (Fig. 2f, Video S1). This is to support the peel and prevent any tearing along fragile layers such as podzol fibres or thin loam bands. If the peel is heavy, for instance in the case of very structured soil, it can be partly folded over the top of the wooden board. Covering the edge of the wooden board with a thick towel can then reduce the risk of tearing that can occur in fragile layers.
Digging out the lacquer peel can be easy and straightforward if the peel is small and does not contain roots or concretions. Very small profiles (e.g. 40 by 40 cm) can even be done by a single person. Extracting a more typically sized peel (e.g. 30 cm wide by 120 cm long) is not necessarily difficult, but it can be arduous if layers are densely rooted or structured. Still, 15 to 30 min is usually sufficient to remove peels from a soil face.
The extracted soil peel can be mounted on a wooden board either directly in the field or after transporting the peel to a laboratory, shed, carport, or garage. Mounting the peel in the field allows for safer transport, yet it does typically mean that the size of the wooden board and thus the final size of the lacquer peel is predetermined – unless there is a possibility of bringing power tools to the field to trim a board to size. Using a fixed board size is not a problem when making soil peels for teaching or outreach collection, but when using peels for soil art it can be worthwhile to determine the final peel size after extraction. After all, since the peel is a mirror image of the soil face (as discussed in Step 2), its final appearance remains a surprise until it is extracted from its location.
When ready to mount the peel, test its position on the wooden board to decide which features to keep. Measuring how much the peel will extend beyond the sides of the wooden board helps exact positioning once the board is glued. If the peel is too heavy to lift, reduce its weight by removing large aggregates by hand and/or by very carefully removing any large clumps of soil with a soft brush. A brush may also be used to remove loose sand (always stroke sand away in the direction of any soil layering), but only if the glue is fully dry.
Cover the wooden board with some of the
remaining glue (Fig. 2g), making sure to particularly cover its sides and
corners as these are the most vulnerable parts of the finished peel. Use of a
notched trowel facilitates an even spread of the glue, while corners and
sides can be reached by hand using household gloves. Work swiftly as the glue
dries quickly, particularly when the weather is warm (
Lift the lacquer peel up with two people and place it on the wooden board directly in the desired location: as the glue will create an instant grip, changing the alignment of the lacquer peel will be very challenging if not impossible.
Carefully but firmly press the lacquer peel to the wooden board with your fingers. Again pay particular attention to the sides and corners of the wooden board to secure these well.
Turn the peel on its side and release any loose sand still resting on the peel by manually knocking the back of the wooden panel. Repeat until no sand falls off anymore. Keep some excess material from each layer (soil, any rock fragments, large roots) to restore any damaged patches later if needed.
Now that the peel has been secured to the wooden panel, trim it using a sharp (Stanley) knife (Fig. 2g). With one side of the knife touching the side of the wood, cut off all parts of the soil peel that extend beyond the wooden board. Retain strips to make mini-profiles or to test the effects of impregnation with a fixing agent (Sect. 2.7).
In some cases peels may have small holes or damaged patches if glue distribution was not uniform or where rock fragments or larger roots have fallen off. These patches can be easily restored by applying some glue and covering them with the appropriate material for that layer, such as soil particles, a rock fragment, or a large root. This is also the moment where roots can be trimmed if desired using shears or nail clippers. There is no predetermined root length: the final root length is very much part of the artistic freedom and the message that is communicated with the soil peel, if any.
The soil peel now requires some rest in a well-ventilated place to let the glue fully solidify – we ventilate our profiles for a minimum of 4 days. As glue fumes can be rather intense, a garage, shed, or covered dry outdoor location is best for this. Make sure to place the lacquer peel in a horizontal position – placing it vertically shortly after mounting may result in vertical movement of the drying glue and thus distortion of the soil profile.
Many authors suggest impregnating the undisturbed front of lacquer peels (e.g. Huisman, 1980; TNO, 2010) to intensify the colours of the soil particles and secure any loose particles. Our team did that from 1978 to 2010 using a large can of the cheapest hairspray sold at the local pharmacist, applying it 1 week after the soil peels were mounted. The hairspray did bring out the colours more, but once surprisingly produced such dark colours that any colour variation in the peel was obscured. It may be that the formula of the hairspray had changed, but the exact reason for this dramatic colour change was unknown. Since then, we have not sprayed peels anymore and are very satisfied with the original colours. As such, there was no need to find an alternative impregnation material. In the case that colours are weak, spraying with hairspray can be a way to intensify colours, but we strongly recommend testing of results along the entire length of the lacquer peel using the trimmed-off edges of the peel. In that case, turn the profile on its side to knock off any loose particles before spraying and ventilate again for a few days before installation.
After a week of rest when the glue will be firm and odourless, the finished lacquer peel can be installed in its final location. Hooks screwed into the top of the board allow it to be hung vertically on a wall in a classroom, office, living room, museum, or wherever this piece of science art is desired. If desired, slats can be used to construct a wooden frame around the finished lacquer peel.
We have heard reports of people annually impregnating their soil peel with spray to “maintain its colours”. We have never seen a need for this and do not perform any maintenance of the finished peels. After changing from lacquer to glue, preservation of our peels has improved such that even intensive use in hands-on teaching does not degrade the peels anymore. If required, dust can be carefully removed from between any roots using a vacuum cleaner set at its lowest speed.
High participation in the maker-ed and DIY movements (Holtzman et al., 2007; Atkinson, 2006) indicates renewed interest in making things at home, while the potential of visualization is being recognized in science communication and education (Evagorou et al., 2015; Venhuizen et al., 2019). At the same time, there is increased interest in the value of soils for life (Keesstra et al., 2016; FAO, 2015). The creation of soil and sediment peels combines all these aspects, and can be done by non-specialists. Materials including glues are readily available at hardware stores, and even novices can create beautiful peels. Here we discussed the benefits of using peels and the challenges posed by the old methods (e.g. Voigt and Gittins, 1977; Van Baren and Bomer, 1979; Bouma, 1969) used to create these peels. We described the main steps of making a soil peel: impregnation of a smooth soil face with glue in the field before extracting the peel and then mounting it on a wooden panel. Because of a technological advance in the impregnation material (going from lacquers to glue), the method reported here is safer, simpler, more successful, more durable, and more accessible because (1) the glue can be used without the use and mixing of toxic chemicals in the field, (2) the firmness of the resulting peel is such that additional support materials (such as cheesecloth) are not required, and (3) consequently the soil peel will last for a long time, even when intensively used in hands-on teaching. While this method can be applied to a range of moisture contents and sand textures, further research on the best environmental conditions is required for those interested in achieving perfection in terms of appearance. Similar exploration is advised for alternative glues. Such additional research would be valuable for some (e.g. soil museums), but based on our experience, we believe that those simply interested in capturing a beautiful snapshot of soils can do so with the more qualitative guidance described in this paper. We hope that this thoroughly tested successful and simple method will inspire and enthuse researchers, educators, and the general public to make soil lacquer peels and thereby bring the value and beauty of soils to a wider audience.
Data used in this article can be found in Sect. 2, Table 2 and Video S1.
Video S1 can be downloaded free of charge from the TIB-AV portal (
Instruction video showing how to make a soil peel in the
field. The supplement related to this article is available online at:
CRS conceptualized the main ideas, with input from JHJC and GP. GP developed the methodology, with input from CRS and JHJC. Visualizations were made by LAGMvdW, who coordinated the video with help from JHJC and CRS. CRS wrote the manuscript with contributions from all co-authors.
The authors declare that they have no conflict of interest.
Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by Wageningen University and Research.
Cathelijne R. Stoof, Jasper H. J. Candel, and Laszlo A. G. M. van der Wal dedicate this paper to educator Gert Peek (now retired), whose enthusiasm for soil science teaching and outreach has inspired thousands of students and staff. We thank Stephan Mantel (ISRIC-World Soil Information) for sharing information about the numerous institutes around the world that ISRIC has supported to make soil monolith collections, which greatly contributed to creating the global overview of locations with preserved soil profiles listed in Table 1. For any additions to this table, please contact ISRIC, who will be keeping updates. We would additionally like to thank two anonymous reviewers for their constructive comments, and a large number of others for their help in sourcing the data in Table 1: Albert Bos, Alberto Orgiazzi, Alejandro Becerra, Alessandro Samuel Rosa, Bernd Andeweg, Bram te Brake, Christine Morgan, Coen Ritsema, Colby Moorberg, Darya van Tienhoven, Erin Bush, Franciska de Vries, Hayley Craig, Ichsani Wheeler, Jacqueline Aitkenhead-Peterson, Jacqueline Hannam, Jakob Wallinga, Jantiene Baartman, Jerry Maroulis, Jetse Stoorvogel, Karen Vancampenhout, Keiko Mori, Kim Cohen, Kirsten van der Ploeg, Liam Heffernan, Marcos Angelini, Maurica Fitzgibbons, Meredith Steele, Michael Strickland, Mickey Ransom, Mirzokhid Mirshadiev, NSSC-GRU, Nynke Schulp, Piotr Pacanowski, Rachel Creamer, Richard Bardgett, Richard Kraaijvanger, Royal Eijkelkamp, Steffen Schweizer, Sytze van Heteren, Wieske Wentink, Wouter Thijs, Wouter van Gorp, and Zhanguo Bai. We thank Niels Kijm for assistance with making the lacquer peel for the video instructions, Bob Czaja and Ann Youberg for discussion of the types of glue that are internationally available, and Bison International for information on the characteristics of BisonKit. Cathelijne R. Stoof has received funding from the European Union's Horizon 2020 research and innovation programme under Marie Skłodowska-Curie grant agreement no. 706428.
This paper was edited by John Quinton and reviewed by two anonymous referees.