Splitting igneous Basalt using wedges and shims.

["David J. Holmes" <wolfscrag@zzzzzzzzzzz>]

Hi Sam,
    There are two basic things you have to deal with when splitting stone.
One is the type of stone and you folks in NZ don't have the best for general
splitting. Oamaru limestone is probably the closest to the term splittable
as it has some sedimentary layers but nothing like granite.
Basalt has little or no grain and is structurally similiar to the hard
Taranaki andesite. Coromandel might but I have never tried worked with it.
To split it you need many holes, 100 - 200mm apart.

I don't know the standard metric sizes of the range of feathers and wedges
you have but not having "cleavage" grain requires more and they don't need
to be that big. If a stone is going to split, it is the even light hammer
blows wedge to wedge that will split the rock not heavy blows. The
continuity of the tone of the "ring" will tell you if you are right.
Slowly and evenly. The old timers used to let the stone "rest" once the
wedges were well set.

The second issue is this. Most commercially produced feather and wedges
are made at 8 - 10 degree angle on the wedges. As less than 50% of the
stone I work with is commercially quarried ( perfect 90 degree cleavage )
most being basalt and other very hard natural stone, it has been an
education, to say the least.
In order to split these stones in the strange configurations that carvers
sometimes need you need the wedges to put the most pressure well into or at
the bottom of the hole. The 8 - 10 degree f & w's are purposely made to
split granite slabs with perfect cleavage with the pressure on the surface
of the slab. I could not get Trow and Holden ( our major stone tool
supplier) to make them so I forge my own wedges at about 5 degrees but still
use the factory feathers. They work well and mild steel rod forged for the
wedges is  alright as they don't shatter from use. Most of the wedges I use
are 3 1/2" to 6" long , 3/8", 1/2",5/8" and rarely 3/4" in dia. Again, it
has a lot to do with what you are after for an end result.

Although small in number, I see that there are a number of hard stone
carvers on the list so am sure they will comment as well from their own
specialization requirements.
Good luck.

David Holmes
Maine USA