Cleaning : What is dirt and why clean ?

Issues

Cleaning absorbs more effort and money and does more damage than any other part of conservation. No other branch of conservation poses more difficult problems, yet no other branch attracts less competent personnel. Contractors often perceive cleaning as easy and profitable. Clients don't know what they want, or why they want it. Specifiers have contradictory technical sources to draw on. English Heritage and Historic Scotland have adopted very different attitudes to the whole subject.....

There is no agreement on "what is dirt ?" : Perhaps the best definition comes from 'Science for Conservators' (originally published by the Crafts Council)...."Dirt is material in the wrong place."

Why clean?

Joe Public, when asked, always say they prefer clean buildings to dirty ones. But then Joe Public reads tabloid newspapers and watches soap on television, so why should their opinion be taken seriously? Do brain surgeons consult in the street to justify their interventions?....

Are there any serious reasons for cleaning buildings?

Some dirt is highly disfiguring. The only way that I can cope with this bizarre streaking (on the north porch of the Saxon church at Bradford on Avon) is by mentally refusing to see it. Just as an inappropriate repair can make a building look absurd, so can visually ludicrous random staining.

On a detailed scale, the same applies. The original carver of this 'stiff-leaf' capital probably intended it to be painted....we don't know. But we do know that it was never intended to be partially covered by soot from Victorian industry.

Before : please pass me the garlic, there appears to be something from the un-dead zone just above your left shoulder...

After : Cleaning can work near miracles in recapturing the form of delicate sculpture.

Visual effect is not the only reason for cleaning. Dirt often carries corrosive pollution, hides dangerous defects and increases thermal stress.

'Biological dirt' is not only disfiguring, it can be very damaging to health. Moulds on damp wall surfaces shed spores that are serious lung irritants.

The power of graffiti to outrage gives it a special place among 'dirts'. Offence, and the giving of it, are both perfectly valid reasons to clean. It is easy to underestimate the fear and loathing that can be generated by graffiti. What would I feel if it were my name and minority group receiving public abuse?

Quite disproportionate efforts are devoted to its removal. Sadly, much of this effort is technically ignorant. Graffiti is often removed without regard to minimising substrate damage It is also a wonderful illustrator of a universal cleaning problem.....how quickly does re-soiling take place? Is graffiti a problem in itself or a symptom of other problems? Is treating symptoms a rational use of resource?

By no means all graffiti is two dimensional. If cleaning is justified, surely the cleaning process has to be extended to merge with repair....Which links the issue of 'why clean?' neatly to the reverse issue of 'why not clean?' Cleaning is often justified, but it is rarely possible without 'collateral damage' to substrates.

Why not clean?

Cleaning at its simplest is...washing the dishes. Clearly defined dirt is removed from a clearly defined substrate. The substrate is unaffected by the process, but is very doubtful if this is ever achieved in building cleaning.

This tower and porch illustrate the problem. The new render on the tower is clean. The 20 year old render on the porch is not. In fact the porch render is almost invisible. It is entirely covered in a layer of mould and algae. This biological layer is not on the surface, it is on and in the surface. Most building material surfaces are textured, porous and permeable. Dirt penetrates the surface and can only be fully removed if part of the surface is also lost.

At this point a review is clearly called for :

....as there is no universal definition of dirt, how can you decide what to remove?

....since building materials have textured, porous and permeable surfaces that are usually damaged by cleaning, how do you determine the level of damage that is acceptable?

....when buildings like this have multiple painted surfaces, quite possibly with 20 superimposed layers, what is the real surface?

.............!

Assuming it is possible to make a rational decision on how to clean, to what and from what, another issue presents itself....how long will it be before re-soiling takes place? Will the cleaning process affect the substrate and make it more likely to attract and hold dirt? Ten years of road traffic has substantially re-soiled this heraldic work....was the cleaning worth the effort?

A well known bank in Cirencester. It is again looking distinctly grubby around ten years after one of the most savage abrasive cleaning campaigns ever unleashed against a building. Is the cost in substrate damage worth such a short lived gain in cleanliness?

Uncovering 'cans of worms'. Most cleaning operations start on the assumption that everyone will be happier when all is revealed. This is frequently not the case. Here cleaning has revealed (with stark hilarity, possibly black humour) that previous repairs were moulded in black mortar. The repairers did so to match the dirt on adjacent surfaces. On removal of the dirt, the mortar is still black because the pigment was an integral part of the mix.....and the mortar was based on Ordinary Portland Cement and is too hard to cut out without damaging the original stonework.

Loss of definition :

The way we see and appreciate carved detail is complex. Most was probably painted when it was first created, and the paint would have been used to accentuate form. Now that the paint has gone, we have to rely on shadow to define form....and on dirt. Over-cleaning destroys form.

Before : The parish loved the 'patina' of the church porch, though they did not realise that they did.

 After : "You don't know what you've got till its gone". Cleaning can easily destroy an image of antiquity.

"There was an old lady who swallowed a fly, I don't know why she swallowed a fly, pr'haps she'll die. She swallowed a spider to catch the fly......". Some cleaning is not started as end in itself, but rather to address other errors and problems. Here consolidation of the door mouldings left them with a shiny 'skin'. Air abrasion was used to remove this. Does such cleaning falsely legitimise flawed conservation techniques?

Cleanliness is next to Godliness : The vast bulk of building cleaning is not done in the name of conservation. It is called housekeeping, and is done by janitors, vergers and householders by the thousand.....

......It is usually done without specialist knowledge of the fabric they are cleaning or the tools they are using. The damage is slow and insidious, but relentless and uncontrolled. A plague on the heavy vacuum cleaners that are exacerbating delamination of these ledger slabs.

The white areas are thick accumulations of water repellent lichens. They are tough and highly resistant to abrasion. The black areas are a mixed layer of carbon and stone that has been attacked by sulphur gasses in the environment. This layer is moderately water soluble. The pale yellow surface of the stone has reacted with pollution to become calcium sulphate...which loses its mechanical strength if wetted.

The puzzle....is it possible to clean one surface without destroying those under or beside it?

Even on simple objects where the substrate is fairly uniform, the nature of the 'dirt' changes over short distances. As you move down the pot, the micro climate changes and so does the biological colonisation. Birds perch on the rim and defecate on the upper slopes of the pot, introducing complex dirts onto the surface.

Again the puzzle.....is it possible to modulate any cleaning strategy with sufficient precision?

A final caution to anyone minded to clean :

Most dirt is both in and on the substrate. The choice when cleaning is not 'should I remove any substrate?', but 'how much substrate shall I remove?'

Even if you are happy with your definition of dirt (and in this case can withstand assault from the massed ranks of mycologists) cleaning is technically difficult, physically demanding and terminally boring, all at the same time. You may have to look a long time to find anyone capable of turning good theory into actual site practise.

Strategies

How to clean :  

How cleaning can be achieved with least damage....

Air abrasion

Shot blasting or throwing rocks......

abrasive is picked up in stream of moving air and impacted against the surface to be cleaned.

The effectiveness, and the potential dangers, depend on

   * the nature of the dirt,

   * the nature of the substrate,

   * the energy put into the abrasive,

   * the angle between the abrasive stream and the work surface,

   * the shape and hardness of the abrasive.....

   * and above all on the skill of the operator

Probably the biggest advantage of air abrasion is that it is a dry (or relatively dry) process.

The most obvious problem of air abrasion is that softer parts of the substrate can be severely eroded before harder parts have been cleaned. In other words, the area being cleaned tends to include varied dirt and substrate and the process tends to exaggerate these differences. Operator skill is the only real defence against this problem.

A detail of the damage that can be done. The surface texture here is entirely the creation of the air abrasive cleaning process.

Here also, the surface may be clean....but it is not the surface that originally existed. The roughened texture will almost certainly mean that re-soiling is very rapid.

A thick carbon crust has been removed from this stone capital but every care has been taken to clean lightly and selectively. However, the long term of the abrasive impact is unknown. There is a real risk that even the gentlest air abrasion may cause micro-fracturing below the cleaned surface.

These pillars have been gently air abrasive cleaned, but illustrate another problem with the technique. Air abrasion will remove dirt from one section of a stone only to deposit it again on immediately adjacent areas. The effect is often called 'gun shading'. All too often the re-deposited dirt has to be removed by water washing.

Dissolution

Water, acids, alkalis and surface active agents (and, shhh!, wire brushes). If cleaning with air driven abrasives has dangers, surely it is safer to use nonabrasive methods.....

Cleaning with water is intuitively safe. Good clean water must rank near to motherhood and apple pie in terms of unassailable virtue. Nothing could be further from the truth. A gaping hole in the knowledge base of building conservation is our lack of data on how water moves within buildings.

If you run large volumes of low pressure water down the outside of buildings, it certainly removes water soluble dirt. (It also removes insoluble dirt that is attached to a soluble substrate.) But what happens to the water that penetrates into the fabric of the building? Where does it come out? Does it cause damage as it exits?

At this well known Yorkshire Minster, the west front was low pressure water washed....

 ....and the carved detail of the west door fell to pieces. It is impossible to be certain that the water washing directly caused the loss of the carved detail on the doorway. However, it is very possible that there is a causal connection. Water introduced at high level has exited through the carving causing much stress as it did so.

Internal water movements triggered by cleaning are only half the story. Water cleaning produces surface run-off that can cause extensive damage lower down the building....

 ....Handling surface run-off is particularly problematic when exposed decorative detail (like this terra cotta) is much more susceptible to water damage than the plain stone walling. In order to clean large areas of stone, very high volumes of water have been used above this string course, greatly increasing the loss of its surface.

All cleaning methods are at their most problematic when applied to materials that have thin surface layers. Any over cleaning or run-off damage will reveal a core that may be neither pretty nor durable. (There is debate over the durability of terra cotta once it has lost its 'fire skin'.)

From the cleaning contractors point of view, water is seen as being too gentle and too slow. Contracts can be greatly accelerated if the water can be combined with brushing and with acid.

Favourite among acids is hydrofluoric. It will certainly dissolve most dirts. Indeed its all too happy dissolving the silica base of brick and sandstone.

Advocates will fairly say that hydrofluoric acid cleaning is a safe technique when performed skilfully.....but mistakes there do seem to be aplenty. Acid was used to dissolve dirt on this brickwork. The dirt is gone but a formidable problem of salt crystallisation remains.

On the same building, cleaned with the same techniques, a different problem. Before cleaning, the building was dirty, but it was not green. It is likely that the acid cleaning has altered the whole chain of biological colonisation on the surface of the brick. Whether it will ever return to a colour we associate with brick is an open question.

Acid cleaning to remove graffiti has left this surface severely streaked. The blocks are terra-cotta, and the clay from which they were made, contained substantial quantities of iron compounds. The acid cleaning agent has reacted with these and selectively bleached them. The problem is particularly acute here because of poor control of acid run off from higher areas.

Stains too difficult even to talk about. Metallic corrosion produces some of the difficult cleaning problems, particularly where the whiteness of a stone is part of the architectural design. There is a firmly established conservation tradition of using strongly alkaline 'poultices' to remove such stains. Time alone will tell whether the disease is sometimes better than the cure. The combination of water and strong alkalis is no less potentially damaging than water and strong acids.

Captive abrasion

Throwing rocks and pouring on water both come with a price. Is there no other way? Indeed there are many other approaches to cleaning. 'Captive abrasion' is here used to describe a whole family of techniques. At the most delicate a diamond burr, similar to a dentists' drill can be used to grind off areas of dirt. Used carefully, such tools are very delicate and controllable, but also very slow.

Much more commonly used are large industrial grinders. Forget all those conservation worries about removing the dirt but preserving the surface....Remove the surface as well...Its so much quicker and cleaner !

 Bricks cleaned with a shameless angle grinder The surface loss is immediate and patterns left behind are very distinctive, but the level of damage is probably no greater than that of harsh air abrasion.

This bank was cleaned with a grinder, but the problem of distinctive circular marks was solved by mounting the grinding machine on a small railway truck. The grinding head was thus held far more firmly and a smooth surface was produced. However good the resulting surface, few would recognise this technique as conservation. Note also that the building has very significantly re-soiled within ten years...was the surface loss worth it?

Lasers

I've seen the future and it glows very bright indeed ! In all the vale of tears that is building cleaning, there is one ray of brightness (!).

Laser cleaning is in its infancy. It is still very expensive and very slow when applied at 'whole building scale', but in the right circumstances it would appear to offer unequalled selectivity and delicacy.

It is very likely that laser cleaning will become the standard by which other cleaning is judged.... However, the basic problems remain : What is dirt? : What is the real surface? : Re-soiling : etc., etc.

The future: lasers are at their best when removing clearly defined, dark coloured dirt from pale substrates.

 The future : even the most delicate sulphate skins have here remained undamaged by laser cleaning. It is inconceivable that any other method of cleaning would have achieved this delicacy.

Sterilisation : The Ministry of Works Dream

A last vision of the potential futility of cleaning. The pursuit of absolute and final solutions to any of the problems of conservation is probably a dangerous waste of time.

The Ministry of Public Building and Works, guided by Frank Baines, made a heroic attempt to stop all decay, forever, on its abbeys and castles. Biological decay drivers (such as ivy) were particularly targeted. Has Sterilisation worked? Can it ever be achieved? Was the attempt worth the cost?