Contamination has plagued archaeobotanical studies involving the analysis of microbotanical remains. Many studies have attempted to understand the nature and sources of contamination that can occur both in situ and during post-excavation processing in the laboratory. Contamination is defined here as starch that can be added to samples through aeolian (airborne) processes or through transmission by direct contact. Laurence (2013; Laurence et al. 2011) provides an extensive review of modern airborne starch contamination that derives from food processing factories such as flour and maize mills, as well as starch that is contained in pollen. Pollen starch is undistinguishable from reserve starch and can be released when pollen ruptures either on the ground or mid-air (Laurence et al. 2011:215). Both insect and wind-pollinated species can produce pollen starch which provides energy for the pollen tube (Baker and Baker 1979). Microscopic slides placed close to sites in Texas where earth ovens were being assessed for ancient starch contained quantities of starch after 96 hours.
Other researchers have recommended that archaeological studies should incorporate an assessment of environmental airborne starch contamination in the field (Loy and Barton 2006; Messner 2011). Yet others recommend sampling sediments surrounding artefacts to assess whether residues on tools are accurate representations of use or contamination (Barton 2007, Loy et al. 1992; Williamson 2006). This is determined through the taxa and quantities present in either sample. This is problematic for a number of reasons. Fullagar and others (1998:51) argue that the: “...difficulty in comparing starch grains on tools with starch grains in sediments is the arbitrary choice of a quantity of sediment for a comparison with a given quantity of residue”. Similarly, as pointed out earlier, starches in sediments can be more prone to enzymatic degradation, so false negative results can be the outcome of these studies (Laurence et al. 2011; Zarillo and Kooyman 2006). It is also difficult to argue that starch found in sediments is contamination or vice versa, if consideration is given to the fact that an area may have been used for food processing and that a tool was left in the same location after use.
Transmission of starch through handling and equipment is another source of contamination in the field. This has been addressed in a number of studies, particularly those
43 concerned with residues on artefacts. Handling of an artefact both during original use and post- excavation processes can cause starch to become attached to both working surfaces and other areas of the tool that are not directly related to use (Barton 2007; Loy et al. 1992). Modern handling contamination can be avoided by wearing starch-free gloves when bagging artefacts into new zip lock bags in the field (Barton et al. 1998:1233; Hart 2011), but often analyses include artefacts that have been excavated without these protocols and so researchers have sought a means to test for levels of contamination. Suggestions to control for contamination include testing various locations on the artefact, proposing that use-related starch is concentrated around working edges (Barton et al. 1998; Loy et al. 1992; Piperno and Holst 1998). Another control is to compare artefact use-wear with the results of starch analysis (Allen and Ussher 2013; Barton 2007; Ussher 2009). Where these are complementary, this provides strong support for food processing technology.
Airborne native starch can not only contaminate sediments in situ or during fieldwork, but also within the laboratory along with starch found in chemicals or through manual transmission. There have been a number of published studies highlighting potential sources of starch contamination in laboratory environments (Crowther et al. 2014; Laurence 2013; Laurence et al. 2011; Loy and Barton 2006; Loy et al. 1992; Messner 2011). The most extensive of these is that carried out by Crowther and others (2014), where not only the consumables and equipment in the laboratory were tested for the presence of starch, but also surfaces and air within various working spaces. This was designed as a comparative study between ancient starch laboratories at the University of Calgary and Oxford University, and the results strongly suggest that starch contamination is a major concern that needs to be addressed before interpreting archaeological material. Consumables were tested by sampling liquids or exterior swabs, while environmental samples were taken using horizontal ‘passive’ traps, or vertical stationary or mobile adhesive traps. High numbers of wheat (Triticum sp.), maize (Zea mays) and potato (Solanum tuberosum) starch were found on particular brands of powder-free gloves, pipette tips, paper towels, and within Calgon and Sodium polytungstate commonly used for heavy liquid separation (Crowther et al. 2014:86). Spaces that had significant quantities of starch included fume hoods, floors and work benches, but these numbers were lessened after cleaning.
Crowther and others’ (2014) study corroborate data from previous studies. Powder-free gloves have been also been tested for starch contamination by Laurence (2013; Laurence et al. 2011) and others within the medical sciences (Campbell et al. 1984; Makela et al. 1997; Newsom and Shaw 1997; Wilson and Garach 1981), and found to have varying quantities of maize starch. These gloves are manufactured in the same factories as powdered gloves, and therefore can easily be subjected to contamination by airborne starch if care is not taken. Similarly, Newsom and Shaw (1997) conducted a survey of airborne starch within a hospital
44 environment and found that an average of 13.8 granules/30L air was present even in areas such as intensive care units. Many of the starch granules observed within these studies displayed signs of morphological and physicochemical modification such as cracking and gelatinisation (Crowther et al. 2014:101; Laurence et al. 2011:228), indicating that the condition of starch cannot necessarily be taken as an indicator of its age.
Protocols that limit the potential for modern starch contamination in the field and in the laboratory have been developed in response to these studies. There are no alternatives for powder-free gloves, but some brands have little or no starch contamination and should be selected over others (Crowther et al. 2014). It is also possible to limit the amount of contact gloves have with samples by using equipment such as sterilised forceps. Limiting or excluding the use of paper products can also reduce the amount of airborne starch within the laboratory environment. All centrifuge tubes, mixing rods, Petri dishes, sieves and pipette tips should be treated for decontamination with 5% sodium hydroxide (Crowther et al. 2014:102) or hypochlorite (Laurence 2013: 52; Laurence et al. 2011) prior to use. These protocols are also replicable in the field, where particular powder-free gloves should be worn when handling samples, and contact with the sample should be limited before placing in zip lock bags. All sampling equipment at the very least should be cleaned with boiling water before use, but disposable items should be utilised where possible. Food should also not be consumed on site (Loy and Barton 2006:165).
Spatial differentiation for various tasks within the laboratory is also essential. Any processing of modern starches for reference collections or experimentation should be carried out in a different room to that where post-excavation processing of sediments is usually undertaken (Crowther et al. 2014; Loy et al. 1992). This is especially important if samples are being dehydrated and then milled or pounded. Consumable and environmental contamination should be monitored on a regular basis to ensure the risk of contamination is low during different stages within the working week or seasons. Monitoring of airborne starch in the field is also a useful measure to at least provide a gauge of the most common modern morphotypes and quantities of starch within the local environment (Crowther et al. 2014; Laurence et al. 2011; Loy and Barton 2006).