9.6.7 HSC Feedback

2001 e)

EVALUATE means + / - and judgement (with criteria) – ie want to restore the artefact to as close to original condition as possible without further degradation. So for each step say the pros and cons. And when there is a decision to be made (ie a criteria/evaluation) say why eg mechanical vs acid concretion removal & stripping vs electrolysis of corrosion.
Remember to store in water first (why), leach (why – and NB leaching only removes SOLUBLE salts) remove concretions (here is a change to discuss pros and cons of mechanical vs chemical removal), electrolysis (pros and cons – cons mainly in terms of current and rate of electrolysis vs surface damage)

WHENEVER discussing electrolysis ALWAYS say a) It removes INSOLUBLE SALTS unlike leaching, b) It draws anions out of the artefact (eg Cl-) c) It restores the metal surface (ie metal ions reduced back to the surface)  AND d) WITHOUT FURTHER DAMAGE. ALWAYS use equations for the anode and cathode AND for the cathode reactions make sure you show the correct corrosion product being reduced (Fe(OH)Cl, Cu(OH)Cl, Ag₂S or PbS). NB in basic solutions the anode reaction will be 2OH^- => 1/2O₂ + H₂O + 2e^-

2002 cii)

Chemical procedures = leaching, concretion removal with acid, corrosion product stripping with citric acid/thiourea (Cu or EDTA (Pb), electrolysis and H-furnace (Fe) – make sure you show equations for the ones you chose.

If you have t speak about a specific artefact with chemical treatment choose the cannon over the anchor.

2003 e)

ASSESS = INTRO, +ve, -ve, judgement

INTRO = define electrolysis (perhaps brief history Davy/Faraday)

+ve = a) It removes INSOLUBLE SALTS unlike leaching, b) It draws anions out of the artefact (eg Cl-) c) It restores the metal surface (ie metal ions reduced back to the surface)  AND d) WITHOUT FURTHER DAMAGE.

-ve = trickier but   it requires energy, time, cannot remove concretions, cannot remove some oxides (eg why H-furnace is used for Fe artefacts sometimes)

Judgement = Has it developed artefact restoration? – ie before this could surface be restored? Could insoluble salts be removed?..

2004 aii)

Wood can be degraded and oxidised but not corroded (metals corrode)

2005 di)

The wax is a BARRIER to oxygen and electrolyte (water/humidity) preventing corrosion

2006 c)

The coins were encrusted (see diagram) thus you can’t just say that Cu artefacts wont have concretions

STEPS

1) Keep wet to avoid crystallisation of salt(and show what happens with equations)

2) Leach – Na2CO3 to prevent dissolution of Pb or Zn & Sn from Cu alloys

3) Concretion removal – with ACID for coins and don’t want to dameg surface engravings – perhaps only dilute/weak acid too else bubbling might damage surface

4) Leaching again?

5) For the Pb you might want to strip with EDTA as the surface is not important BUT for Cu the surface IS important (coins) thus you probably wont strip off the corrosion products as you want to RESTORE the Cu in the corrosion back to the surface

6) Electrolysis of coins – (and Pb?) make sure you use the correct equations – see notes for Q 2001e) above

VERY important – electrolysis removes corrosion NOT concretions

2010 a)

Don’t forget that the metal bands are part of the artefact and would be corroded!

2011 bi)

You need to know that the main corrosion product on silver is Ag2S and thus must show it being reduced back to silver (not just silver ion reduction)

2012 a)

Because the artefact material was not specified you should give SPECIFIC problems that can occur if salt crystallises in wood/leather, ceramics and metals. DON’T FORGET that acid can form on metals if salt crystallises and make sure you can write the equation for this.