Bleaching fossils with heat

[Denis Arcand]

Rocks are handy for a campfire. A common use for stones in a campfire is to create a protective ring around the flame. This ring serves two purposes: Creates a barrier between the fire and the rest of the forest, keeping the rest of the woods safe. Creates a wind barrier that can help light the fire, especially in wet and windy conditions

 

Whatever the reason, someone made a campfire at my fossil dig site, it's a small secluded beach along a river. A nice place to make a campfire. By the way, I noticed that a rock they used to make the protective ring around the flame, had very distinct and visible small white fossils on it. A small chuck of rock found in the ash contained even whiter fossils. I deduced that the bleaching of fossils is created by the heat of the fire.

 

 
I started experimenting, I put a rock containing fossils in the oven and set the heat to maximum (don't laugh, this is a very serious experiment) after heating the fossil for a solid hour, nothing happened. For my second try, I used my barbecue (don't laugh), I think I can have a higher temperature with it, I cook the same fossil for an hour and I get the same result: nothing. I needed more heat, so for my third try I put the fossil not on the grill but directly on the burners, and hooray I got white crinoids and Sowerbyella that really stand out from the matrix. Does anyone know what is the chemical process that creates this amazing effect?

 

I wonder if this is a good method to give a second life to dull, unappealing and scientifically unworthy fossils. The heating process helps to bring the fossils out of the matrix, making it visibly more interesting. What do you think?

 

 

A dull and unattractive fossil I used for my test

 

 

After high temperature heating

 

 

[DPS Ammonite]

 

I wonder if the heat killed and carbonized encrusting organic matter such as lichen. The loose carbon then fell off. Sort of like a self cleaning oven does.

 

Try bleach first to see if it whitens the fossils. 

Edited July 7, 2022 by DPS Ammonite

[FranzBernhard]

Maybe it also burned (oxidised) carbonized microparticles within a thin surface layer of the fossil? Try to chip off a piece of one of the whitened ones.

 

Anyways, be very careful when you are subjecting rocks to great heat. Some can explode due to thermal stress.

 

Franz Bernhard

[TheInvertebrateGuy]

7 hours ago, Denis Arcand said:

Does anyone know what is the chemical process that creates this amazing effect?

 

scientifically unworthy fossils.

^ (not true)

The fire used the iron oxides from the rock as fuel. Eventually the rock will oxidize again over a long period of time.

Edited July 7, 2022 by TheInvertebrateGuy

[JohnJ]

Another thought would be micro fracturing of the calcitic fossils causing them to reflect light differently. 

 

Using protective eyewear, I heated (less than 30 seconds) a piece of calcite with a propane torch and immediately observed the formation of thousands of stress fractures.  The result was a 'whiter', more opaque look.

 

Heating in a campfire is probably slower than using propane, so the amount of micro fracturing might be greater.

 

The bottom piece was not heated.

The longest piece is about 3 cm.

[FranzBernhard]

9 hours ago, JohnJ said:

Another thought would be micro fracturing of the calcitic fossils causing them to reflect light differently. 

I think, you got it! Great! 

Franz Bernhard