Integrated Assignment 04

Restoration of historic monuments

The context of this assignment shows students that science is relevant to traditional crafts, as well as modern methods. Students investigate two important factors in the preparation of traditional render for coating stone walls – the salt content of the sand, and the strength and surface texture of the hair added as reinforcement. The latter aspect relates directly to their study of composites in Materials for making things (Applied Science Unit 2).

To undertake the assignment successfully, students should have prior experience of procedures for identifying anions and cations, silver nitrate titrations for determining chloride content, testing tensile strength, and the microscopic examination of hair.

To save time, students could work in teams, distributing the tasks between themselves, with each student or pair developing the procedure for one aspect of the work. At the end they must share their results in order to agree their joint report for the National Trust.

Practical tips

Salt in sand

A range of salt contents could be ensured by mixing sea sand and builders’ sand in various proportions. Students should be encouraged to perform a semi-quantitative analysis by simply comparing the amounts of silver chloride precipitated during the anion tests.

If the quantitative analysis extension is to be undertaken, teachers are advised to determine the salt content in advance. Standard procedure SP 0008 is designed for 1 g samples containing 1% to 3% sodium chloride. Sand samples outside this range will require appropriate adjustments to the sample size in order to obtain reasonable titres. Extraction with nitric acid is not required. Wash out the salt with distilled water, and titrate the whole filtrate.

Since the nitric acid is not required, and the sample solution will be more or less neutral, the double titration method is not essential. Some students could titrate the filtrate directly against standardised silver nitrate solution, using potassium chromate indicator. The results of the two methods could then be compared.

Strength of hair

In this context (as reinforcing fibres in a composite) it is the breaking load that matters, rather than the extension. Therefore, it does not matter that the hairs used are not as long as the 300 mm specified in the standard procedure. However, the hairs do need to be long enough to clamp and attach the load.

For short hairs, another method of attaching the load may be needed – for instance, sandwiching the hair end between pieces of paper (to provide more friction) and clamping with a strong bulldog clip, on which masses can be hung.

Microscopic examination

All students should prepare a microscope slide of a hair or fibre. These could then be passed around to compare different samples. The importance of labelling should be stressed. High magnification is required to study surface texture. Students should be able to appreciate that the purpose of using hairs is to reinforce the render and hold it together. They are therefore looking for hairs with a rough surface that will grip the mortar matrix.

Students could investigate whether transmitted or incident light is better for viewing the surface texture.

As an extension, they could also examine grains of the various sand samples, to compare angularity. The smoother and rounder the grains, the less well they interlock in the mortar matrix.

Health and safety note

Silver nitrate solution stains skin and clothing black. The stain is metallic silver and will not wash out. Protective clothing and disposable gloves should be worn.

oncentrated hydrochloric acid is corrosive – but only a few drops are needed for cleaning the flame test wire.

Apparatus and reagents

• Standard Procedures:
  • SP 0001:2005 Methods for setting up and adjusting a light microscope
  • SP 0002:2005 Methods of preparing slides for microscopy
  • SP 0006-1:2005 Tests for anions
  • SP 0006-2:2005 Flame tests
  • SP 0008:2005 Method for determining the salt content of butter
  • SP 0012-1:2005 Methods of testing strengths of materials

For salt in sand

• access to balance
• dish for each sand sample
• filter funnel and papers
• conical flask to collect filtrate
• sample tube or small bottle for each sample
• 4 droppers (unless reagent bottles have built-in droppers)
• distilled water wash bottle
• dilute nitric acid (approx 1 mol dm^-3)
  Caution: corrosive
• silver nitrate solution (approx 0.1 mol dm^-3)
  Caution: stains clothes and skin black
• ammonia solution (approx 1 mol dm^-3)
• watch glass
• nichrome wire with small loop at one end
• concentrated hydrochloric acid
  Caution: corrosive

For optional chloride titration

• access to balance (preferably weighing to 0.001 g)
• filter funnel and paper
• 2 x 250 cm³ conical flasks
• 25 cm³ pipette and pipette filler
• 50 cm³ burette
• 0.025 mol dm^-3 silver nitrate solution
  CAUTION: stains clothing and skin black
• standardised 0.025 mol dm^-3 potassium thiocyanate
• 50% ammonium iron(III) sulfate solution (50g in 95 cm³ water + 5 cm3 5 mol dm^-3 nitric acid)

For testing tensile strength

See diagram in standard procedure SP 0012-1

• clamp and stand
• wooden blocks
• bulldog clip
• metre rule
• dowel
• strong thread
• 100 g mass hanger
• 100 g masses
• 10 g masses

For microscopic examination of fibres:

• microscope with low and high power objectives
• light source (lamp or good daylight)
• various hair and fibre specimens
• microscope slides and cover slips
• disposable dropper
• forceps
• mounted needle
• mountant (e.g. DPX Caution: contains xylene [dimethylbenzene] – harmful, irritant) Available form BDH Chemicals (VWR International, Poole, BH15 1TD 01202 669700 www.vwr.com

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