The online resource for the historic environment

3.05 Materials

To assist your understanding of this Unit see BS 7913:2013
para 4.3: "The Assessment of Significance" - a), 1) and 2)
para 5.3.2: "Materials"
para 6.3: "Assessment of Performance and Pathology"
para 6.7: "Proven Techniques"
para 6.10: "Some Common Repair Issues"
Section 7.0: "Maintenance"

“The drop of rain maketh a hole in the stone, not by violence, but by oft falling.”
Hugh Latimer c.1485 - 1555

The science of materials, their use, historical development and patterns of weathering and decay is too vast a subject to go into within this unit, however, some simple statements on the subject may assist your understanding about what to include in your portfolio of evidence for accreditation and, hopefully, get you thinking.

You should be able to demonstrate skills necessary to undertake a condition survey of the asset, of its materials’ defects, identify their causes and make balanced, well constructed decisions and plans for intervention that will most effectively deal with the problems. You should be able to demonstrate an understanding of materials use, limitations, physical and chemical properties, interactions both beneficial and detrimental with other materials, patterns of behaviour and decay, etc.

Draw up a list of materials that you might encounter in a building of any age in the region where you live/work/practice.

(>30 excellent, >20 very good, >10 good, <10 think harder)

Draw up a list of the properties of materials that control the way they perform in the built environment.

(>30 excellent, >20 very good, >10 good, <10 think harder)

Examples of properties in this context are strength, density and coefficient of thermal expansion, but not mass or colour.

Materials decay for a variety of reasons including (on a non-exhaustive list basis) the following:

  • Weathering (various processes here)
  • Adverse interaction with other contiguous materials
  • Excess loading
  • Inappropriate design and detailing
  • Limitations of materials use
  • Limitations of life expectancy
  • Effects of damp (corrosion, dissolution, decay, mould growth)
  • Effects of fungal and insect damage (biological change)
  • Differential movement
  • Mechanical damage
  • Natural deterioration (see Second Law of Thermodynamics: all processes tend towards decay and disintegration)
  • Natural degradation (sunlight on organic materials for example)
  • Simple failure
  • Natural faults in materials

A few definitions of terms might also be useful at this stage:

  • Deteriology: the study and prediction of the deterioration of structures and its effect on service life
  • Pathology: The identification and diagnosis of defects
  • Defect: a short fall in performance
  • Degradation: deleterious effects of sunlight for example on organic materials. Also defined as the conversion of a complex molecule into simple fragments
  • Durability: ability of a material to perform its function over a period of time
  • Failure: termination of a material to perform a required function
  • Prognosis: the consequence of a defect if it is left untreated.

(after Douglas, J.E.H)

The three main causes of defects might be summarised as follows:
  • dampness
  • movement
  • biological change
    (Addleson & Rice, 1991)

BS 7913: 2013 para 6.3 "Assessment of Performance and Pathology" also lists "Common Defects..."and expands on Addleson and Rice's list of three main causes

The ‘sources’ of defects may, however, be more difficult to identify, viz:

  • Dampness may have as its source groundwater, rainwater, condensation or services leakage.
  • Movement may have as its source changes in moisture, temperature or chemical action.
  • Biological changes may have sources in moisture-induced change, fungal attack, insect attack, corrosion or sulphate attack.

BS 7913: 2013 para 6.3.3 notes: "In some cases defects are caused by a series of harmless alterations that are damaging in consequence". (The sometimes 'insidious' nature of change)

“Thus the main aim in any asset defects diagnosis is to identify correctly the actual agency or source.”
Douglas 2001.

You might consider, as examples, deterioration through biological changes in a timber-framed structure and deterioration due to chemical changes in a load bearing wall construction. You should identify why these changes are occurring (causes), and likely sources of the causes of damage you have identified, together with prognosis of effects of deterioration and likely repair or remedial response.

Let’s take pathology: diagnosis relies on facts about state and condition but how does that establish the position of the observer of those facts assembled by whatever means. With due respect to Dickens and Mr Gradgrind (and Arnold). “What I wants is facts…Facts alone are wanted in life.” The tale of interpretation is taken up by Arnold in Reading Architectural History when the author suggests “First ascertain the facts then draw your conclusions from them.” But is that the right interpretation, after all any interpretation of the facts must, by very nature, be subjective. Arnold goes on to say that “…First get your facts straight, then plunge at your peril into the shifting sands of interpretation.”

Extending the philosophical sojourn, C.P. Scott advises:“Facts are sacred, opinion is free.”

The moral of this discursive interlude might be contained in Earl’s commentary in Conservation Philosophy and Practice that advises, in respect of the pitfalls of interpretation:

“…a sound philosophy is not based on a set of immutable rules but a clear understanding of what one is setting out to achieve. Comprehensive knowledge of all the relevant facts…will not in itself point the way. The practitioner…must develop a critical and self-critical frame of mind, nurturing the ability to proceed from facts by way of logical argument to defensible – if not inevitable – conclusions.”

Summarised, perhaps, by - draw your own conclusions, based on the facts resulting from detailed investigation, but be willing to defend/modify them!

You should therefore be able to demonstrate by adoption of the above process an ability to identify the cause and source of defects in materials. You should also be able to make a prognosis or prediction of how each individual cause and source of defects may affect the overall historical asset; how the defect may affect performance and longevity and how maintenance might be planned to rectify or reduce the effect of the defect without loss of authenticity; with minimum loss of fabric and with the principle of reversibility actively considered as part of the intervention process.

“There is an understanding in conservation work that imperfections of age are not necessarily translated as ‘deficiencies’
Bell, D (2001)

A material may be subject to decay but it may not be necessary to replace it simply because it demonstrates decay. You must be able to assess the rate at which decay is occurring, make an assessment about when it is likely to become necessary to undertake repair or change the detailing to prolong the life of the degrading material. Such repairs should, where possible and practical, be reversible. This to ensure that, (should future technology offer a better solution to protecting the material and the building’s significance), earlier action may be removed and an improved response made.