By Caterina Celada-Prior
Textile conservation is becoming more aware of the environmental impact of current conservation practices, and professionals are searching for greener and more sustainable alternatives. Within this context, during the summer of 2020, I explored the potential of natural plant-based surfactants for the wet cleaning of historic textiles as part of my dissertation for the MPhil Textile Conservation. The research aimed to provide a groundwork for making informed decisions in the application of saponins as a potential alternative to current surfactants in textile conservation treatments.
Wet cleaning is an important part of the domestic care of textiles and textile conservation practices. Traditionally, before the mid-20th century, soaps and natural surfactants (non-synthetic) were the main sources of cleansing agents. These were gradually superseded by the arrival of synthetic surfactants (highly efficient and affordable).
SURFACTANTS IN TEXTILE CONSERVATION
Synthetic surfactants—commonly called detergents—lead wet cleaning treatments in textile conservation. In order to provide a controlled cleansing action whilst minimising changes and damage to the fibres, the selection of surfactants is subject to their stability. Mostly, synthetic anionic and non-ionic surfactants are chosen due to their specific properties: lack of colour, neutral odour, low critical micelle concentration (CMC), good wetting power and high solubility at low temperatures, amongst others. However, these purely synthetic refined oil-based and petroleum-based surfactants raise environmental, economic and social concerns related to their production (e.g., atmospheric emissions and intensive farming) and waste disposal (e.g., biodegradability).
In alignment with a sustainable conservation practice, natural surfactants such as Funori and saponins have been researched as potential alternatives to current practices. Saponins (from Latin ‘sapo’, i.e. soap) are amphiphilic molecules (mainly found in plants) with surface-active properties and soap-like behaviour. These especially draw my attention as historically they have been widely used around the world and across cultures for cleaning textiles. Unfortunately, despite the “re-discovery” and rise in popularity of soap nuts (Sapindus Mukorossi) as a green alternative to commercial detergents, the rich intangible heritage attached to their use is at risk of loss.
Knowing of the existence of Saponaria (Saponaria Officinalis) in Spain as a traditional means for personal hygiene and washing textiles (a tradition that today is almost forgotten), along with a personal interest in traditional and sustainable conservation practices, these traditions piqued my curiosity regarding the potential role of saponins in textile conservation.
SAPONINS IN TEXTILE CONSERVATION
Through a literature review, I came to understand the extent of their use, the types of saponins and their sources. I could also verify whether saponins’ properties were fully understood and if they would be appropriate to use in textile conservation.
My literature search disclosed a historic, steady—though fairly inconspicuous—presence of sources on saponins within the profession. Inherited traditional beliefs indicating saponins to be mild and therefore adequate for delicate textiles seemed (in many cases) to govern the rationale behind their application. Details on saponin sourcing and rationale for use were occasional. The most recent uses in conservation are mainly done using purified forms. However, I also found references to natural saponin plant extracts being used in the early days of the profession and continued use still in some areas of the world.
The promising usefulness of saponins in conservation is illustrated in the incipient research focused mainly on detergency parameters of purified forms of Quillaja Saponaria (Soapbark tree) and Saponaria Officinalis (Soapwort). However, the literature shows a limited understanding of these compounds (detergency properties, behaviour, sources and limitations of their use) and their implications for health and the environment. Some groundwork was necessary to fill in the missing gaps in order to inform textile conservators’ decision making in the application and development of further research into these compounds.
Diving into the understanding of saponins, I found them to be widely varied and complex compounds. Although chemical structures affect detergency properties across species, saponins as pure compounds have many similar characteristics to their synthetic non-ionic counterparts. They have good wetting properties, a low CMC and foaming and emulsifying abilities, having the potential to be efficient and effective non-ionic cleaning agents within conservation standards. Establishing what species of saponins would be the ideal cleansing agents still requires further research. Even so, factors such as high aggregation (i.e. number of surfactant monomers in a micelle) and hydrophilic-lipophilic balance number (HLB), could be indicators of higher cleaning power, with monodesmoside saponins (i.e. saponin species containing one sugar chain) presenting better foaming abilities.
Some species of saponins are commercialised as pure compounds for biomedical and industrial purposes. These are the main forms recently used in conservation due to their controllability and similarities to powdered forms of synthetic surfactants. However, the complex purification processes required come with elevated costs, making them inaccessible to many.
In contrast, I became particularly interested in the potential behind traditional forms of saponin extraction (plant extracts) and use. By reclaiming sources locally available and independent of industrial processing and commercialisation, plant extracts could become an eco-friendly source of surfactant. I also saw this as an opportunity to expand the scope of preservation to the intangible heritage linking the knowledge of the environment with the preservation of textiles.
Saponin-yielding plants as sources for detergent are abundant including a wide variety of plant families worldwide. These often receive their common names in reference to their soap-like properties, e.g. Soapberry (Sapindus Saponaria). I was able to identify almost fifty different plants referenced in the literature as traditional means used in the cleaning of textiles around the globe proving local sources are readily available, and there is the potential for sourcing diversification. Ivy (Hedera Helix) and Yuca de Mojave (Yucca Schidigera) are two examples of native sources, the former from Europe and the latter found in the southern United States and Mexico.
CHALLENGES IN USING SAPONINS
The use of purified forms of saponins allows for accurate calculation of detergency parameters (e.g., CMC) in washing solutions and keeps preparation time low, while the use of plant extracts obtained by traditional means of extraction (i.e. maceration and decoction) comes with challenges and conundrums regarding their stability for application in contemporary conservation practices.
Plant extracts linked to the knowledge of local sources of surfactants require only basic processing and implements, making their use accessible and affordable regardless of available infrastructure. Plant extracts contain saponins and many other substances such as flavonoids (colourants). Plant extracts also vary their saponin content, both in their chemical composition and their concentration according to several factors. These include plant species, parts of the plants included, geographic area of growth, development stage of the plant and post-harvest treatment. The presence of compounds other than saponins raises concerns in regards to the stability of these extracts (e.g., ageing in textiles) according to conservation standards, and the variability of saponin composition and content diminishes the precision of their application.
There are obvious positive environmental advantages of saponins as a renewable surfactant due to their high bio-degradability as surfactants—both for the end-products and by-products such as shells, leaves or roots—as well as the possibility of diversification of sourcing crops, avoiding monocultures.
That said, we must bear in mind that “natural” does not automatically equate with “innocuous”. A mindful practice involving health and environmental protection should always be encouraged, no matter the source of the product.
Due to saponins’ biochemical defensive function in plants, once they are separated from the plant source, they can alter the media or inadvertently affect other living organisms. Saponins can be highly poisonous for fish, insects, worms, molluscs and amphibians. In warm-blooded animals (including humans), saponins can often be toxic in direct contact with eyes and blood due to their haemolytic properties. Avoiding their ingestion and implementing the use of PPE (protection of eyes, skin and direct contact with open wounds) would ensure safe use in conservation. The disposal of saponins into waste management systems containing water treatment facilities has been reported as safe due to their high biodegradability in this context. However, their disposal to open sources of water should be avoided, and caution should be taken when watering fields with saponin-containing water.
Saponins in their pure form (isolated from other compounds in the plant) can be considered a potential source of surfactant for use in conservation in line with standard practices. That said, we should recognize the need for further research regarding the long-term and ageing behaviour of fibres treated with saponins, particularly in their unprocessed extracted forms. The relationship between plant colourants and possible staining on fibres also needs to be considered when identifying the best sources of saponins as cleaning agents and their cleaning performance.
In addition, the implementation of saponins in conservation practise should be accompanied by a rise in experiment and treatment documentation standards. Accurate reporting of substances used (i.e. the source of saponin, form and preparation) is undoubtedly essential for informing decision-making and future research.
Caterina Celada-Prior graduated from the University of Glasgow (UK) in 2020 with an MPhil in textile conservation following an MA in conservation from the University of Barcelona (Spain). After acquiring experience through several internships in Spain, the UK and Switzerland, she now works as a textile conservator at Zenzie Tinker Conservation Ltd, Brighton, UK.
(Read the entire article in the April-May 2022 "News in Conservation" Issue 89, p. 12-16)