thinning spalls and flakes with a pyramid cross section?

[ozy clint]

gday guys, i was having trouble today with some flakes i was trying to turn into knives. they came off the rock in flakes about 2" wide x 5" long. one side was flat the other was peaked. like a pyramid in cross section.

how do you go about thinning these things? i tried taking flakes off the flat side hopeing to make it the same profile as the peaked side. but all i did was take beautiful thinning flakes off and it stayed flat.

do you have to take flakes off the peaked side? this would mean heaps of lost width.

thanks for you help.

[chessieboy]

Take and set up a platform on the base and run a flake up the ridge.  It may need to be repeated but it will flatten the stone up.

[aaron]

Good to see you are working on the rock i sent! post pictures!
With a spall like you describe, ( a turtle), you have 3 options:
1. like chessieboy said, prepare a platform at the base or tip and try to run a flake along the length of the peaked side.
2. like you tried originally, flake the flat side until it matched the peaked side- you have to take short flakes by hitting with a "glancing blow", striking straight down. The problem with this is that you loose width fast.
3. The Jim Riggs way- a.k.a. "uniface-biface". With this technique you ignore the rule that says platforms should be at or below the center line. You use a "high" platform and flake off the ridged side. Set up a strong platform and use a nice hard, straight-in (not glancing) blow. This should send a deep flake over the ridge, removing much thickness. It's kinda counter-intuitive, but it works. The benefit is that you do not sacrifice much width. This generally only works with percussion.
Aaron

[aaron]

see also http://www.primitivearcher.com/smf/index.php/topic,27281.0.html