Articles Conservation

Examination of the decay pattern and cleaning interventions on historical marbles

3 January, 2020 9 min reading
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Based on the thesis by: Christina Antonopoulou, Chemical Engineer | Materials Scientist | M.Sc. in Structural Analysis of Monuments & Historical Constructions

Examination of the decay pattern and cleaning interventions on historical marbles

The main aim of this thesis, by the author Christina Antonopoulou, is to analyze the decay pattern that could be presented on a historic marble and also the potential cleaning interventions on it.

Definition of the marble

In modern times the word marble is characterized by various interpretations. Geologically speaking, marble is defined as the transformed carbonate rock, that is formed by the recrystallization of carbonate rocks, under high temperature and pressure, at great depths in the earth’s crust. In the marble industry, as marble is referred each compact rock with a hardness of 3-4 in the Mohs scale, which additionally can be cut, polished and used as decorative or structural material. This compact rock also includes limestone, breccia and conglomerates but also onyxes, serpentines, travertine and granite. Depending on bedrock marbles are divided into limestone, whose main mineralogical phase (> 50%) are calcite (CaCO3) and dolomite, where the main mineralogical phase (> 50%) are dolomite microcrystals.

 

Properties of the marble

Color:

The fundamental minerals forming the marble is white and therefore the clean marble is white. Apart from the white, marble also appears in off-white or gray color, while rarely found in shades of red, green, yellow or even black. The difference at the colors is due to the impurities – particles at the granules – that absorb light of different wavelengths. The carbonaceous substances confer the gray color and often responsible for the blue background. Moreover, the red color is caused by hematite, the green shades by olovine or chlorite and last but not least the yellow one is made by limonite, biotite and phlogopite Finally marble is characterized as milky when light could be reflected at the surface and appears as nontransparent

Texture:

Tissue describes the size, the formation, and the way that the crystals are connected, while texture shows their spatial distribution and their general appearance of the rock. The tissue of marbles can be characterized as granoblastic or suture tissue, either simple or complex. The granoblastic one presents equal-diametric, polygonal crystals and granural view in cleavage surfaces. At the regular tissue, crystals present an irregular shape, while at the complex one, crystals form bifurcations that penetrate at the invaginations of the adjacent crystals. The two types of suture tissue create smooth and in cleavage surfaces. The texture of marble is generally isotropic and compact, however, there are times that laminar or ribbon-like texture is observed. In this case the crystals are arranged at the symmetry axis.

 

Physical and Mechanical Properties:

Even if marble is a metamorphic rock, has very similar properties with those that its primary minerals have, which means with calcite and dolomite. The most important of the physical and mechanical properties are:

 

• The apparent density of the marble range from 2670 – 2770 Kg / cm3;

• The water absorption coefficient is 0.1- 5%;

• The elastic modulus is equal to 65000 MPa;

• The compressive strength is equal to 70-150 MPa;

• The resistance to wear friction is equal to 2,5 – 6,0mm;

• The impact strength is equal to 0,4 – 0,7 m;

• The thermal expansion coefficient is equal to 0029 – 0,0047 mm / mo C;

• Thermal conductivity is equal to 2.47 – 2,73 W / mK.

 

Chemical Properties:

As exactly happens with the physical properties, the chemical properties of the marble are similar to those of calcite and dolomite. The calcareous marbles are easily soluble in dilute acid solutions, leaving an insoluble residue of silicates and graphite. The dolomitic marbles are not attacked by dilute acid solutions, but only by hot acid solutions. It is also observed that at 25o C the Mg2+ could be substituted by Ca2+.

Generally, the deterioration of the marble surface is adjusting to the general environmental conditions. The main factors that control this phenomenon are the air pollutants, humidity, rain water, winds, solar radiation, soluble salts, fluctuations in temperature, bacteria, fungi, algae, and birds.

 

Mineralogical composition

As it is already mentioned in the definition, the marble may be derived from parent material either calcite or dolomite. Apart from the composition, both differ in physical and chemical properties as it is going to be clear from the following table.

 

 

The density and the refractice index are greater in the case of dolomite, while, calcite reacts with acid solutions and stained blue after boiling for 5 min with (CuNO3)2, and violet in solution 0.1% alizarin – S 15N HCl, in contrast with dolomite which maintains its color in both of the abovementioned cases. Specifically, for the fundamental minerals of the marble:

 

Types of deterioration on marble surfaces

The most common types of deterioration on marble surfaces are divided into five basic categories:

  • Chemical deterioration on marble surfaces;
  • Surface crusts;
  • Deposits on marble surface;
  • Deterioration resulting from previous incompatible methods;
  • Biological deterioration.

 

Principles, necessity and strategy of Cleaning Interventions

Principles and Guidelines for Interventions

In the case where an examined site is considered as a decayed site, there is a sequence of steps that has to be followed. Firstly, it has to be selected a specific and detailed intervention approach and procedure and secondly we have to determine the remedial measures that have to be taken under consideration. Each and every decision relevant to the conservation plan should be taken according to the respect to the present state of the building and the heritage importance that characterize it. The general available resources for facing the necessary repairs and also the financial budget should play a decisive role into the procedure of deciding the intervention plan. The final action plan has to include all the different aspects that create the final picture of the monitored site, which means architecture, structure and last but not least materials.

 

Conservation Principles sets out a method for thinking systematically and consistently about the heritage values that can be ascribed to a place. People value historic places in many different ways but ‘Conservation Principles’ shows how they can be grouped into four different categories that are the following ones:

• Evidential value: the potential of a place to yield evidence about past human activity;

• Historical value: the ways in which past people, events and aspects of life can be connected through a place to the present – it tends to be illustrative or associative;

• Aesthetic value: the ways in which people draw sensory and intellectual stimulation from a place.

 

Communal value: the meanings of a place for the people who relate to it, or for whom it figures in their collective experience or memory.

As far as concern the main consideration in developing and linking recording documentation and information management practice there is a procedure that have to be followed. To be more specific, there are different approaches in the Conservation of a monument that are defines by the International Council of Monuments and Sites-ICOMOS.

The above-mentioned approaches include:

• Maintenance;

• Preservation;

• Restoration.

Cleaning methods

1. Natural methods

1.1. Water based methods:

  • Spraying with low pressure water;
  • Mist;
  • Steam;
  • Atomic water.

 

1.2. Mechanical methods

  • Dry sandblasting;
  • Wet sandblasting;
  • Micro-sandblasting;
  • Dry ice blasting;
  • Abrasive blasting (Wheatgerm);

 

  • With ultrasound;
  • With tools.

 

1.3. Cleaning with special clays

 

2. Chemical methods

2.1. With acids, bases, detergents

2.2. Paste Mora, AB57

2.3. Ion-exchange resins

2.4. Ammonium hydrogen carbonate and ammonium carbonate

2.5. Cleaning with biological pastes

3. Thermal methods

3.1. Laser- Light Amplification by stimulated emission of radiation

 

4. Methods of cleaning against bio-erosion

The prevention and control methods of biological agents could be categorized into direct or indirect. Indirect methods last longer and prevent or inhibit bio erosion, but they are not easily applicable in the open environment of monuments. Direct methods are short-termed methods and require repetition at regular intervals.

 

The appropriate methods are selected each time depending on:

• the type of biological development;

• the type and extent of the surface cleaning;

• the deterioration levels of the surface under maintenance;

• the best cost-risk ratio/benefit.

 

Sometimes the best solution is achieved by the combined application of indirect and direct methods.

 

A. Indirect Methods

The control of environmental factors in the open environment of monuments is very difficult and usually not applied, despite their long-term effectiveness. It should be briefly mentioned that:

• Reduction of moisture with covers-canopies. Affects the growth of all the microorganisms, mosses, hydrophilic plants, etc.

• Reduction of temperature with covers-canopies. Affects all organisms.

• Reduction of the effect of sunlight with covers-canopies and of artificial light with reduction of lighting time. Affects photosynthetic flora and micro-flora.

• Reduction of nutrient substrates (organic – mineral crusts) with cleaning interventions. It mainly affects heterotrophic microorganisms.

 

B. Direct Methods

Direct methods aim at the sterilization of micro-flora or higher plants during the intervention, therefore, the surface becomes a substrate for new biological growth. So, this is a short-term treatment against bio-erosion and this is the reason why these interventions should be repeated periodically. These methods are either selective which means that they destroy the living cells, or non-selective which means that destroy living matter as well as any other biological substance and so they are considered as simple cleaning methods.

 

Mechanical methods: These methods are effective for the removal of plants, algae, lichens, etc. and their most important characteristics are the following ones:

• Development of intense mechanical action, especially when there are plant roots into the walls;

• They are followed by removal of authentic material;

• They are considered as effective for a short time.

 

Natural Methods: In this method, where there is use of electromagnetic radiation wavelengths in the ultraviolet spectrum (10-380nm) and microwaves (1cm-100mm). UV rays destroy nuclease acids and above a certain threshold are fatal for any kind of organism and they have been used for algae, lichens, cyanobacteria and green algae. Even if this method is considered as simple and there is no use of toxic substances, there are also some disadvantages that are the following ones:

• Low invasive ability.

• When the surface is naturally pigmented or painted, it has a destructive effect on pigments.

• The method should be applied with specific preventive measures: operators must wear mask and goggles

• The system is difficult to be transported to the site and is very expensive.

 

It has proven a very useful method for wood and cellulosic materials due to the biocidal activity on microorganisms and insects. The application of microwaves has been experimentally applied in agriculture. Overheating from microwaves inhibits seed germination but also causes damage to the stone. Finally, low frequency electrical systems were applied in order to drive away birds from the sites.

 

Biological methods: These methods are selective and exploit the nutritional peculiarities of microorganisms and plants, or the biological competition of various species to achieve the disappearance of their unwanted growth.

 

Chemical methods: In methods biocides are used. The parameters to be taken into account for the selection of chemical methods are:

• The maximum specific toxicity • Indirect toxicological characteristics and effects;

• Environmental pollution;

• Interaction with the substrate;

• Effectiveness;

• Cost.

 

The most common chemicals used are:

• Neutral triazine compounds with low water solubility for the decontamination of plants;

• Quaternary ammonium compounds (Desogen), copper compounds with ammonia and copper complexes with hydrazine in order to control algae, cyan algae and green algae;

• Hypochlorite lithium, Lito 7, Lito 3 (trade names) for mosses and lichens;

• Ampicillin for fungi and actinomycetes.

 

It has been shown that organic solvents such as methanol, ethanol and glycerol help the development of seeds and mushrooms.

 

Criteria for the evaluation of a cleaning intervention

  • Criteria No 1: Chemical Mineralogical Composition
  • Criteria No 2: Texture, morphology and surface coherence- Surface Micro-structure;
  • Criteria No 3: Color
  • Criteria No 4: Preservation State- Decay Susceptibility- Time Durability

 

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