Observations on Current Usage of Conductivity in Cleaning Carpets from Historic Houses by Jonathan Tetley

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Copyright Jonathan Tatley

After many years of being confronted with big, dusty, sooty and stained carpets to clean, it became apparent that Jonathan Tetley needed more information at hand to decide on cleaning treatments, which were mostly undertaken with reference to his experience or that of other conservators.
Since the 1980s, conservation requirements for institutional bodies such as The National Trust (UK) have included carpets as textiles worthy of consideration to be conserved, and not just to be consigned to storage or treated as sacrificial items. My work at the Tetley workshop over the past 30 years has been to develop and refine our cleaning treatments to meet these requirements; careful choice of cleaning methods is not only important because of irreversible results, but also because it is quite likely the piece has not been ‘properly’ cleaned for up to 200 years or more, and may not be cleaned again for a similar period of time.
Environmental conditions of the historic setting where the object is kept may have certain inbuilt problems such as temperature changes, extreme fluctuating levels of humidity, foot traffic or handling. It may be that some or all of these conditions cannot be changed or ameliorated, or that the piece may have become acclimatised to conditions very different from those when it began its life. This would require returning the piece to a semblance of its former condition, if clean.
Since 2007 I have adopted an approach to the treatment of carpets, that is largely influenced by the work of Dr Richard Wolbers, the paintings conservator and conservation scientist based at The University of Delaware in the USA.
This approach introduces the use of conductivity and pH testing as a means of both monitoring and controlling the inherent condition of the object. Through monitoring the conductivity of both the carpet and the cleaning solution, it is possible to adjust the conductivity of the solution to match the desired resting conductivity of the cleaned carpet. This stabilises the carpet for the conditions to which it has become accustomed, making it less likely to take up atmospheric and environmental pollutants in reaching equilibrium. There can be a risk from ‘overcleaning’ with just deionised or demineralised water where the piece is left in a highly conductive (ion-attracting) state. As Rebecca Pavitt points out in her workshop review in News in Conservation, October 2012 ‘Cleaning of painted surfaces – Wolbers strikes again!’:
“The ability of a material to conduct electric current is related to the concentration of ions in solution. Every material has some amount of ionic compounds on its surface and, in the case of porous materials such as paper and textiles, within its body. This can be measured by taking samples…and using a…conductivity meter.”
Textiles, particularly large hairy carpets, have things happening both at the surface and within the body of the textile. There can be accumulations akin to geological strata: gravy and canapés, underlaid with wine and essence of dog, Devonian sands or granite grit and limestone mud forming dunes in the weave in addition to the acidic conditions inside the yarns. Clearly the accuracy of readings that a paintings conservator can achieve with a micro conductivity meter can only get us so far with carpets. In addition, there may be risks of potential or actual dye run; some or all of the dyes in the piece may be loose or unstable to certain aqueous or other solutions, or to light. Previous repairs have to be considered, not only as potentially runny, so in need of testing along with the original dyes, but also in terms of how they will affect the structural stability of the piece during cleaning.
Because of these different factors, I have developed varied approaches to testing. Where there are loose yarns, it is possible to test both for the conductivity and pH of the whole piece and of individual dyed yarns. Samples are macerated and immersed in control water, then tested after 1–2 hours. Where the carpet is intact, and removing yearns is damaging, control water is passed through one area, collected and tested. From this initial conductivity and pH testing, various solutions can be constructed to match the conditions of the piece.
I have noticed that historic carpets often have a particular smell and a low pH, often between 3.0 and 5.0. This could be attributed to years of coal smoke and other airborne pollutants. In addition, acidity produced by ageing wool, thought to cause deterioration of cellulose in the linen wefts, can cause small splits that grow into broken areas leaving the woollen warp and knots intact but loose and going into holes. Tests comparing the sorption rates of sulphur dioxide (SO2) of wool with other fibres have showed that wool absorbed SO2 steadily in low amounts over a long period of time. Sulphur dioxide (from car fumes, for instance) when mixed with water (humidity) turns to sulphuric acid, causing acid hydrolysis to break down the polysaccharides of cellulosic, starchy, or hemicellulosic materials to simple sugars. This would seem to indicate that where wool carpets have linen warps or wefts, over time and in damp conditions such as in British historic houses, sulphuric acid will have formed and contributed to the breakdown of the cellulosic fibres.
For instance, in the 1757 hand-knotted Axminster carpet from the drawing room at Dumfries House Ayrshire, Scotland, the smell suggested an acidic condition caused by breakdown of the wool; the exposure of the cellulosic linen warps indicated the necessity of cleaning. Several samples were colour tested with Dehypon® and ROW (Reverse Osmosis Water). Brown dyed samples showed colour run on blotting paper with swabs. DT (Devon Tap Water) tests were then set up.

Quantity pH Conductivity
ROW 2.5 ml 5.4 15
DT 2.5 ml 7.4 103

The carpet was setup to test for actual pH and Conductivity as well as for colour run, using DT.

pH Conductivity
Blue 5.8 115
Black 5.8 104
Brown (Dark Khaki) 5.6 114
Dark Brown 4.6 110
Light Brown (2nd dyeing DB?) 4.8 88
Red 5.0 85
Yellow 5.2 113
Dark Brown from rotten area 5.0 116
Dark Brown from Split 5.2 76

Wash solution included Dehypon LS45 at 0.5g per litre of stock solution.

Colour testing results
Because of the dye run testing with Dehypon and ROW, it was decided to test with two higher pH solutions: -

pH Conductivity
SA05 (& with Dehypon®) 6.0 78 Sodium Acetate @ 0.5g per litre stock
SAL2 (& with Dehypon®) 7.9 115 Sodium Acetate @ 0.5g per litre Sodium Chloride @ 0.0025g per litre Acetic Acid @ 0.0004% stock
DT/DT & Dehypon® 7.4 103

The tests were checked and showed no run with DT and Dehypon®. The object had to be washed in three sections, and it was intended that the same procedures were repeated at each section.
The conductivity and pH of the cleaning solution were 110 and 7.2 respectively (using non-ionic detergents ensures that wash and rinse solutions have the same readings). At the end of cleaning the first section, the conductivity was down from a high point of 2470 to 103 but after the carpet was dry it was still looking slightly dull and the pH was lower than desired at 4.5. This might have been due to the inhibition of the water flow beneath the carpet laid flat, pile down, on the wash bath floor. The areas that had been stabilised for cleaning with netting showed signs of brown colouration, as did blotting paper tests. Since the pH was lower than desired (4.5 rather than around 5-7) and the appearance of the carpet was still dull, the pH tests were rechecked. Although the electronic pH meter turned out to need recalibrating, it was decided that the concentration of detergent was insufficient. It was decided to double the detergency concentration for the second and third sections. Because of this, the second and third sections had final readings of 103/5.2 and 88/5.5 for conductivity and pH respectively.
This case study introduces the issue of detergency and critical micelle concentration (CMC). I believe that excessive concentrations of detergent can be harmful to the piece, and whereas CMC (the point at which optimum cleaning efficiency is achieved) is required in commercial laundry cleaning, it is not always desirable in historic textiles. You only need to look at some of the electron micrographs prepared by Dr Bill Cooke at UMIST to see the drastic changes wrought by strong detergent solutions. However, I put my hand up to having used too weak a detergent solution initially with the Dumfries carpet, which necessitated mid-clean alterations to the concentration of detergent used. In my opinion the current optimum wash strength for tank immersion cleaning is at 0.3g detergent per litre of water. With wet extraction (hand held wet vacuum extraction and spray applied solution), it is possible to reduce this further due to the mechanical action of suction pressure as an additional cleaning factor. I am always aware of the desirability of proper rinsing, and the stronger the detergent solution, the harder it is to rinse out residues satisfactorily.
Colleagues have argued that fluctuations in conductivity and pH during the cleaning, and the disparities within the uncleaned piece itself, make periodic testing a fairly pointless exercise, since no accurate measurements are possible. I disagree, since periodic testing not only provides ‘snapshots’ of the condition of the piece for the record, but also allows a picture to develop of the effectiveness of the cleaning – or not. By a process of ‘progressive approximation’ it is possible to move towards the desired resting conductivity and pH.
It is also possible to reduce dye run by matching conductivity and pH of the cleaning solution, wash or rinse, to problem dyes in the carpet. Since there are parameters with both conductivity and pH that I try to observe, this means that some pieces cannot be safely wet-cleaned, but it is surprising how many can be, even if full immersion cleaning is rejected in favour of partial wet extraction cleaning. The Waterloo Chamber Agra carpet at Windsor Castle has a blue dye in the design of the border, which turns out to be Saxe Blue. This dye showed unacceptable migration during testing and the piece was therefore only wet cleaned in the field and dry cleaned in the borders.
In addition to wet extraction cleaning, I have developed further methods utilising a combination of mechanical action and detergency to effect cleaning. I designed a structure that would enable effective cleaning of an Aubusson carpet from Harewood. These carpets often do not lend themselves to immersion cleaning due to dye problems and have a tendency to ‘tramline’ with wet extraction cleaning, where certain areas of the weave retain more residues than others. The model of the stamp pad suggested itself, where the amount of liquid coming onto the adhesive coating of the stamp can be controlled by the pressure applied to the waterlogged sponge pad. I came up with a bath structure in which sponge pads were placed in the bath section, a measured amount of solution was poured on and rolled in, and then the section had solution drawn through by suction with a water extraction vacuum through a net; the purpose of the net being to hold fast any loose or fragile areas undergoing suction pressure. The monitoring procedures of conductivity and pH were observed throughout.
In conclusion, after five years of working with conductivity and pH monitoring, I would argue that this is probably the single most useful tool at my disposal in planning and executing appropriate methods for treating historic carpets. In finding out the environmental conductivity and pH to which an object has stabilised, I know how I aim to leave the carpet at the end of cleaning. By continual monitoring, I cannot only record the changes that are happening during the cleaning process, but adjust the methods to produce the desired result, and address the problem of acidity in carpets with a cellulosic, or partly cellulosic structure. It is also clear that much greater control can be exercised in dealing with problem dyes, enabling a more informed decision to be made as to what solution to construct or whether to wet clean at all.