Slab Knapping

[Ryan Jacob]

So I have 3 spare glass windows to break. They are all about 1/4 inch thick. Is it possible to knap these with only pressure? If not, I can use indirect percussion.

[bjrogg]

Yes Ryan it certainly is possible to knap them with pressure only. I usually still use percussion, indirect and pressure but there are methods for pressure only.
Bjrogg

[Ryan Jacob]

Bjrogg
What methods are these? He only techniques I know are alternating or zig zag and platform angling. Direct percussion is always a challenge for me using only hammerstones.

[Outbackbob48]

Ryan, check out the Blue Hardin thread I used pressure flaking on this window glass. This pc of glass was done with hand pressure flaker and Ishi pressure flaker and a little edge work with horseshoe nail tool. 1/4" glass is to thin for me to percussion.  Bob

[Ryan Jacob]

Oubackbob48
Yeah, kinda similar situation now that I think about it. How do you use an ishi stick to build more pressure? Also, is it corrected for me to establish an edge first before considering long flake travel?

[Chippintuff]

, or at least half way.To use an Ishi stick: tuck the butt end between your arm and your side; put the back of your holding hand against the inside of your thigh; put the back of your flaking hand on the inside of the other thigh;let your hands keep all the angles correct; pop the flake off by squeezing your thighs together.

Yes. It is best to establish a single edge around the sides by zigzagging before being concerned with the length of the flakes. The second time around the piece, make the flakes a little longer so that convexity is created. On the third pass try to push the flakes at least half way across so that flakes from the opposite side can meet them in the middle removing the flat surface.

WA

[Outbackbob48]

Ryan, first I zigzag edge of glass with very short flakes. You need to build convexity  so flakes will travel further. Raise edge to one side and use Ishi on next pass to increase length of flakes. Raise edge and repeat  with Ishi on opposite face. I try and get my flakes to go passed center on my 3rd pass and remove all flat areas or shiny part of glass. To build pressure with the Ishi you need to press straight in hard and then squeeze with legs while still pushing straight in with hands. This will get your flakes to travel further than with hand flaker. Rember flakes like to travel up ridges and humps , but don't like to travel on concave and flat areas, remember to try and build nice smooth convexity and your flakes will start to travel further. Cross section is rarely talked about but very important for flake travel whether pressure flaking or using a giant dogwood billet. If you ever get a chance to see James Parker knapping ask him about cross section , the man is a master knapper to say the least. I hope this helps and isn't to confusing. Opps didn't see Chippintuff reply so a little over lap. Bob