decay process

chemical factors impacting decay

• pH (acidity levels)

• Salinity

• General chemistry of the site

• Presence of metals

physical factors

• Temperature

• Water movement

biological factors

• Organisms that grow over or degrade the site

• Organisms that consume organic matter

organics - bone

• Can become waterlogged and turn sponge-like.

  

textiles

• Conservation is limited to natural fibers (animal and plant origin):

  • Wool, hair, silk, cotton, flax, jute, hemp, nettle, grass

• Animal fibers (protein) are more resistant to decay than vegetable fibers (cellulose).

• Susceptible to bacteria, light, and microorganisms.

organics - wood

• Microorganisms and wood-boring organisms consume cellulose for energy.

• Wood can absorb up to 850% of its original dry weight in water, leading to waterlogging.

• Covered wood has natural defense against corrosion; uncovered wood is likely to rot.

factors influencing rate of decay f shipwrecks - exposure

1.  increases

   • Sites exposed regularly (e.g., intertidal zones) deteriorate faster than submerged sites.

   • Completely buried sites remain intact longer, especially in fine silt.

     

factors influencing rate of decay f shipwrecks - equilibrium

decreases

• During submersion, objects reach equilibrium with their surroundings.

factors influencing rate of decay f shipwrecks - disturbance

increases

• Rate of deterioration increases when the environment is disturbed:

  • Highly energetic environments (strong currents, shifting sediments)

  • Human disturbance (excavation is a last resort)

silaneous materials

• Glass, pottery, ceramics, porcelain, stone, marble

• Pottery and ceramics generally survive well in marine environments.

• Non-impervious materials (e.g., earthenware) can absorb salts, leading to damage when salts crystallize.

• Concretion can build up over time, and stains (iron oxide, black metallic sulfide) may appear.

  

metals

• Iron, copper alloys (brass, bronze), tin, zinc, silver, pewter, gold

organics

• Timber, rope, bone, wool, cotton, textiles, resins, silk

metals - electrochemical corrosion

• Metals lose electrons to form positive ions, transferring electrons to other metals.

• In seawater (electrolyte), an electrochemical cell forms between two metals (e.g., iron and copper).

• The more reactive metal (anode) corrodes, while the less reactive metal (cathode) remains intact (sacrificial anode).\

conditions for electrochemical corrosion

1. An anode (corroding metal)

2. A cathode (less reactive metal)

3. Contact between anode and cathode for electron flow

4. An electrolyte (solution with ions)

5. A reactant (e.g., dissolved oxygen) at the cathode

ferrous metal corrosion

• Iron corrosion (rusting): $ \text{iron} + \text{oxygen} \rightarrow \text{iron oxide} $

• Requires the presence of oxygen and water.

annerobic corrosion

• Caused by sulfate-producing bacteria in decaying organic material environments.

• Bacteria use hydrogen to reduce sulfates to sulfides, accelerating corrosion.

• Reaction: 4Fe + H2SO4 + 2H2O -> FeS + 3Fe(OH)2

• (Iron + Sulfuric Acid + Water → Iron Sulfide + Iron Hydroxide)

concretion

• Material on the seabed may become encased in hard corrosion products (marine concretions) or colonized by fauna.

• Acts as a physical buffer, but increased acidity can destroy iron objects, leaving only impressions.

• Concretion forms from $ \text{OH}^- $ ions from electrochemical corrosion reacting with calcium hydrogen carbonate in seawater, creating a layer of insoluble calcium carbonate that seals the metal from oxygen.

archeological prinicpals

• Archaeologists apply various principles to interpret archaeological sites effectively.