OPOpenPithMake curiosity visibleChemistry lab / Mineral cleaning

About this bench

Five topics: first know what it is, then decide whether to touch it.

Cleaning a mineral specimen is a chain of chemical choices. The same cleaning liquid removes dirt from quartz and damages calcite. This bench keeps five jobs apart: identifying, dusting, removing iron stain, the selectivity of acid, and shine versus glue. Each topic changes only one or two conditions.

Stoichiometry, Mohs hardness and refractive index are textbook values worked out from formulas. The cleaning speeds are a teaching model. They show trends only and are not measured cleaning rates. For unknown minerals and pyrite, leaving the piece alone and handing it to a specialist is the right answer.

Five findings

Five findings

  • Identify first: a fingernail is about 2.5 in hardness, calcite 3, a steel needle about 5.5 and quartz 7. The needle scratches calcite but not quartz. One 0.05 mL drop of 1 mol/L hydrochloric acid can eat at most 2.50 mg of calcite and gives 0.61 mL of carbon dioxide. Unknown samples and pyrite get no acid and no washing. Their verdict is “leave it and hand it over”.
  • Dust: when 80 % of the dust is loose, 5 brush strokes bring the cover down to 33.4 %, 10 strokes to 22.3 % and 20 strokes to 20.1 %. More brushing never goes below 20 %. The rest is packed in the cracks: rinsing after 10 strokes brings the cover to 0.4 %. When the brush is enough, there is no need to move up to chemicals.
  • Iron stain: after 30 minutes the cover is still 98.5 % in water, 69.8 % in trisodium citrate and only 0.1 % in the reducing system. To make 100.0 mL of 0.0500 mol/L trisodium citrate you weigh 1.4705 g of the dihydrate. One gram of sodium dithionite is 5.74 mmol and can reduce at most 0.917 g of iron(III) oxide.
  • Carbonate and acid: 1.00 g of calcite is 9.99 mmol. Reacting it all uses 19.98 mmol of HCl, which is 20.0 mL of 1 mol/L acid, gives 244.8 mL of carbon dioxide, and leaves the open beaker 0.440 g lighter. With only 10 mL of acid the reaction stops after 0.500 g has dissolved. Quartz does not react. When quartz grows on a calcite base, the base is eaten away and the three crystals fall one after another.
  • Petroleum jelly and glue: light hitting a quartz–air face head on is 4.57 % reflected. With water on the other side it is 0.54 %, and with petroleum jelly 0.060 %. A crack has two faces, so the jelly cuts the reflected light from 8.94 % to 0.12 %, and the crystal looks clear. The 12 mm break is not one millimetre shorter, and a push with a bamboo stick still knocks the top off. Only after it is glued back does the push fail.

Model and sources

Model and sources

  • Identifying: hardness uses textbook Mohs values, and a tool leaves a scratch only if it is harder than the mineral. Pyrite is set to 6 (published range 6–6.5). The acid drop follows the CaCO₃ + 2 HCl ratio. Dolomite barely fizzes in cold dilute acid, so “no fizz” does not prove a mineral is not a carbonate. The triage table is the cautious plan from the research report. It is not a compatibility certificate.
  • Dust (teaching model): each stroke removes 30 % of the loose dust that is left. The brush cannot move the part packed in the cracks. Rinsing is a first order process with a time constant of 1.5 seconds. These numbers show trends only.
  • Iron stain (teaching model): the cover falls as a first order process. The illustrative rates are 0.0005 for water, 0.012 for trisodium citrate and 0.25 for the reducing system, per minute. They show the order only: water does almost nothing, citrate alone is slow, and reducing plus binding is fast. The reducing recipe is a soil analysis method. It contains sodium dithionite, which gives off toxic gas with acid, so only trained staff may use it, under extraction.
  • Carbonate and acid: molar masses are sums of atomic masses, and CaCO₃ is 100.09 g/mol. Gas volume uses 24.5 L/mol at 25 °C and 101 kPa. Mass, acid used and gas volume balance in every frame. The speed is a teaching model: the rate is proportional to the exposed area and to the acid strength, and it stops when the acid is spent. The crystals fall when 60 %, 35 % and 15 % of the base is left. That part is illustrative.
  • Petroleum jelly and glue: the Fresnel formula for head on light, R = ((n₁ − n₂)/(n₁ + n₂))². The indices are 1.000 for air, 1.333 for water, 1.47 for petroleum jelly, 1.49 for Paraloid B-72 adhesive and 1.544 for quartz. The push only tells “falls” from “holds”. No bond strength is given.

The times, bubble sizes, amount of dust and the way the crystals fall are a sketch of the mechanism. They are not real working times. Real specimens are cleaned only after trials on pieces that may be sacrificed, and a conservator decides. This bench does not offer hydrofluoric acid, hot strong acid or acid washing of unknown minerals.

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