A BUILT HERITAGE SERIES: AN INTRODUCTION TO SIMPLE SALT ANALYSIS TESTS AND POULTICE DESALINATION METHODS FOR: SANDSTONE (PART 3 OF 3)

BUILT HERITAGE SERIES: AN INTRODUCTION TO SIMPLE SALT?ANALYSIS TESTS AND POULTICE DESALINATION METHODS FOR: SANDSTONE?(PART 3 OF 3)

Mrs. Emilia Zambri, MSoc Sci Tangible Heritage Conservation.

PREFACE

Welcome to this series of techniques for managing change and assessing the condition of architectural heritage structures and historic heritage fabric. This newsletter aims to provide professionals, including Heritage Consultants, Architects, Archaeologists, and Conservators, with practical and effective methods to determine the most appropriate approaches to management, conservation, and restoration.

Inspired by the challenges encountered in safeguarding historic heritage, especially in under-resourced areas, particularly in Africa, this series has been developed to assist colleagues globally. The goal is to present simple and easy-to-use frameworks, case studies, and tests that can be employed to evaluate the preservation, conservation, and restoration needs of historic heritage, ensuring their safeguarding for future generations.

By utilising these techniques, the objective is to empower heritage professionals with the knowledge and tools necessary to make informed decisions about the preservation of our shared cultural heritage. Join us as we embark on a journey to protect and preserve the historical structures that define our shared identity.

4. ??MONITORING SALT REDUCTION TREATMENTS USING WATER EXTRACTION PROCEDURES

In order to determined and quantify the efficacy of the chosen desalination treatment, it is crucial that a portion of the unused material is also tested using the same analytical procedure used to analyse the other samples. Depending on the circumstance at hand, the analytical process can get highly complicated if the poultice material contains ion exchanging components.?

With this said, knowing how much salt was extracted per unit of surface is important for poultices. As a result, the entire poultice layer and the underlying separating sheets of Japanese paper or an equivalent should be included in the sampling procedure.

4.1.1 ??ACQUEOUS EXTRACTION

In the majority of cases, the purpose of a quantitative salt analysis is to estimate the destructive potential of the salts in the sandstone fabric to help inform further conservation measures.? The extraction of the salts using deionized water and the analysis of the ionic content of the extracted solution is the simplest, most practical, and popular technique for analysing the salt content of porous materials.

4.1.2 ??ACQUEOUS EXTRACTION METHODOLOGY

Despite there being no standardised method to undertake water extractions, I typically choose a representative sample of 5g of the dry poultice material, and mix it with 100 ml of de-ionised water, and agitate it for 30 minutes. Overall, the filtration of these samples takes 10 minutes to 1 hour. Notwithstanding, such materials have a propensity to clog the filter paper during the filtration process, which increases the filtration time. According to my experience, the filtration process often takes far longer for the materials used to make 'blank' poultices as opposed to poultices that have been used to undergo salt extraction treatments.

5.1. ??SALT TESTING METHODS

5.1.1. ??ELECTRICAL CONDUCTIVITY

Electrical conductivity is one of the methods I use to test the salinity of non ion exchanging poultice samples using a salinity meter. Contrary to popular belief, conductivity meters do not provide direct salinity readings, instead, they measure the conductivity of water (mS/cm: S=Siemens, an electrical conductivity unit. mS=milliSiemens), which is why they are called electrical conductivity (EC) sensors. EC sensors are used to test the salinity of water as it records the number of total dissolved solids in the water.

5.1.2. ??ELECTRICAL CONDUCTIVITY TESTING METHODOLOGY

The electrical conductivity is measured by passing an electrical current through the solution and recording the amount of current that flows between two electrodes. Since salts conduct electricity, the readings increase as salinity levels increase. This conductivity is the measurement of electrical flow through the water, which is directly related to the ion (salt) concentration of the water. The objective of this reading is to see the decrease of the total salt content in the solution after each subsequent poulticing treatment cycle.

Below is a total salt analysis of the poulticing desalination treatment on sandstone. As discussed above, this was undertaken using an electrical conductivity sensor.

5.1.3. ??ION TEST STRIPS

In conjunction with the aforementioned test, the additional use of ion test strips can enable time saving, semi-quantitative assessments of significant ions and chemicals in the mg/l range. The use of ion strips is particularly helpful in determining anions as they are cheaper than more expensive laboratory techniques. The range of mg/L concentrations at which an ion can be measured varies depending on the ion and the specific brand of the strip. Chloride, nitrate, and sulphate ions in desalination solutions can be inexpensively, quickly, and semi-quantitatively determined through the use of ion test strips without further sample preparation.

Similarly to pH strips, test strips for other ions are also commercially available. In addition to confirming the presence of anions or cations, they may also estimate its concentration semiquantitatively. These test strips are offered for different ions, such as chloride [Cl-], sulfate [SO4](2-), nitrate [NO3-], nitrite [NO2-], phosphate [PO4](3-), and ammonium [NH4+].

Cations are typically simpler to anions. Sodium (Na+), potassium (K+), calcium (Ca++), and magnesium (Mg++) are the four most prevalent cations. Although I do not advocate for the following, Archaeologists often say that if you "taste" a grain of salt from an efflorescence, you can tell if it is magnesium salt or sodium chloride (ordinary table salt) since the latter tastes bitter. Irrespective, anion strips can also used to identify the type of anion that is present. The strips for chlorides [Cl-], sulphates [SO4](2-), and nitrates [NO3-] are the most practical to have, meaning they are the most frequent anions. Ammonium ion [NH4+] and phosphotes can occur but are not frequently present.

Despite this, the test strips have a number of disadvantages, including their high cost. Thus insitu testing should only be used when required for ion identification processes. Additionally, they can also be applied to a specific sample in the lab for a semiquantitative analysis.

5.1.1. ??ION TEST STRIPS TESTING METHODOLOGY

This article is specifically focused on result outcomes with in a detection range, I am able to provide a semiquantitative concentration determination methodology upon further request.

Following the water extraction method descried above, the filtered solutions are analyzed for their specific salts using ion test strips.

The procedure for the proper use of the strips is:

1. The reaction zones are wetted with the sample solution being tested by dipping;
2. The excess liquid is then shaken off;
3. After the given reaction time has elapsed - a maximum of two minutes - the coloring of the reaction zone is compared with the color scale on the package to determine the concentration and recorded.

5.1.1. ??ION TEST STRIPS TESTING RESULTS

The Sulfate Test Papers have test zones on the strips that contain varying amounts of the red-colored thorin-barium complex. In the presence of the equivalent amount of sulfate ions the color changes to that of the yellow thorin (o-[3,6-disulfo-2-hydroxy-1-naphtyl-azo]-benxenearsonic acid).

The color reaction is red - yellow, and the detection gradations are :

<200 · >400 · >800 · >1200 · >1600 mg/l [SO4](2-).?

The Nitrate Test Papers have test zones on the strips that form a red-violet color when nitrate is reduced and converted to nitrous acid which diazotizes an aromatic amine, this coupled with N-(1-naphthyl)-ethylenediamine to form a red- violet azo dye.

The color reaction is white- red-violet, and the detection gradations are:

0 · 10 · 25 · 50 · 100 · 250 · 500 mg/l [NO3-] .?

The Chloride Test Papers have test zones on the strips that contain varying amounts of silver ions. In the presence of the equivalent amount of chloride ions there is a decolorizing as silver chromate is converted to silver chloride. The concentration of chloride is measured semi-quantitatively by visual comparison with the color scale.

The color reaction is brown - yellow, and the detection gradations are:

0 · 500 · 1000 · 1500 · 2000 · ≥ 3000 mg/l[Cl-] .?

Generally speaking, I use Em Quant Test Strips and have found that the test strips for the different salts have different detection levels and have consequently categorized them at the following low, medium and high levels.

6. ??CONCLUSION

In the context of historical sites, it is best practice to limit sampling to the bare minimum requirement. By validating the presence of anions and or cations in poultice treatment cycles, salt test strips are used to determine the presence of soluble salts on the surface of a structure. Strips can approximate the concentration of ions present, but they cannot identify the actual salt, which requires a laboratory process for identification, such as microscopy or X-ray diffraction. As a further additional step, the total dissolved solids in the sample solution can be measured using a conductivity meter which measures the conductivity of the aqueous extracts from the poultice samples.

Given that sampling techniques are customised for each unique situation, it is difficult to offer broad guidance on what to do. However, some aspects appear to be generally universal and are discussed below.

Before any sampling is undertaken, it is necessary to precisely define the goal of the analysis. Knowing that the "total salt content" or "anoin and/or cations" is not sufficient; one must also understand the particular purpose for which this data is required. This includes defining the analysis's ultimate goal and the questions it will attempt to answer, the type of sampling technique used and the circumstances under which it was carried out, the customised analytical procedure for the laboratory analysis, the limitations of the technique and last but not least, a thorough interpretation of the data that is gathered.