Breaking and shaping stone

Margaret Brooks (1973) once talked to a Yorkshire waller who said he used his hammer only:

…with something of the feeling of giving way, something of a feeling of defeat. One ought to be able to fit all the stones, make use of all the stones whatever their shape and size. One should manage with the stones as they are, especially stones from an existing wall that’s fallen. They have been used before; it should be possible to use them again.

This is a sentiment that many wallers would agree with. Nevertheless, breaking and shaping stones is often a necessity either to provide material of a more usable shape and size, or to trim it to a more regular finished product.

General points

1. Be careful when breaking stones, hammering or drilling. Safety goggles must be worn.
2. Examine each stone before hitting it to see how it will break. Almost every stone, even granite, has some lines of weakness along bedding or cleavage planes, or in areas of frost damage and other weathering stress. It is along these planes that stones break most easily and predictably. If there is no obvious crack to work on, hammer away until one appears and then concentrate your efforts on this crack.
3. To quarry slabs of bedded or cracked stone, enlarge the cracks with a pick until you can drive in wedges with a sledge hammer, or prise the rock apart with a crowbar.
4. Most types of walling stone can be split with a hammer of an appropriate size and weight for the size and type of stone. Hit the stone ‘fair on’ or you will just chip it away and round it off. If you can’t split it as far in as you want, you may have to cut it in several stages, working back gradually to the correct shape. Hit with the full face of the hammer head as shown.
   
   This is the most effective action, causes least wear on the hammer, and helps prevent chips from flying up into your face. Use a full, solid swing except when doing careful trimming. Hit very slightly away from the line of proposed fracture, so that the correct part of the stone breaks off. If the stone doesn’t split immediately, keep hammering away at the dust which you create. The dust acts as a wedge to force the stone apart.
5. Experiment with each type of stone to find the way in which it splits most readily. Sometimes it works better to hit at right angles to the grain, than to strike in the usual way along the grain.
6. Break stones on the ground, not on the wall, which will be disrupted. Don’t break stones in your hand, as you are likely to hurt yourself. Support the stone against another stone and hold it firmly with your left foot. It is easier to split the stone when it is positioned more or less on edge, as shown, rather than flat on the ground.
   
7. Before you break up a big stone, make sure it won’t be needed as a through, cover, topstone or gate post. Try to break it so as to produce several usable face stones, or one useful face stone plus fillings. Don’t waste effort and stone trying to shape stone exactly for normal walling.
8. There are various ways by which stone can be shaped more precisely when required. Cotswold and other soft stones can be chipped away using a hammer with a narrow or pointed pein. Other methods include scoring and wedging apart big slabs, drilling and wedging freestones, and splitting finely bedded or foliated stones with a hammer and chisel. Techniques are described in the following sections for slate, granite, schist and gneiss. These techniques can be adapted for use on other types of stone.

Slate

Slate can be cut in square-cornered blocks of the desired size and split down to almost any thickness using a cold chisel and a lump hammer. A bricklayer’s bolster can be used to break slate across the grain. The ideal chisels are special slate-cutting chisels, but these are hard to obtain. If you do manage to acquire second-hand slate-cutting chisels take care of them, as their flanges are easily broken of if they are misused. A typical pattern is shown below.

Quarrymen often work with a set of three identical chisels, allowing two to rest while the third is in use. The set should be sharpened and retempered about once a month if in constant use.

A block of good-quality slate can be split easily and precisely in two planes, but not all three. It can be cleaved, that is forced apart along a cleavage plane. It can also be split along the grain, which is the original bedding plane which lies roughly at right angles to the cleavage. However, it cannot be as easily split perpendicular to both the cleavage and the bedding. Lower-quality slate, more often encountered in rough walling, can be cleaved, but not so easily split along the grain.

To cleave slate, place the stone on end with its cleavage planes running vertically. Hold the chisel or bolster in one hand, perpendicular to the surface as shown below, and tap it firmly but not heavily with the lump hammer. If cleaving a narrow block, hold the chisel in the centre of the line which you wish to cleave. If cleaving a wide block, start near one end and work gradually along the line.

If the block is too large it may not split, but a crack should appear which you can gradually force apart with further light taps of the chisel or by gently prising with a wrecking bar.

To split slate along its grain, start with the chisel or bolster at one edge of the slab and move it along the line of the desired cut, tapping it in the same way as described above. At first you may have to go over the line of the cut more than once, but with practice you should be able to split it with two or three blows. It helps if the ground below the line of the cut is soft enough to absorb some of the shock and distribute it through the slate.

To split slate perpendicular to both the grain and the cleavage, first cut a groove with the chisel along the line which you hope to split. In quarries special chisels are used, but you can use an ordinary cold chisel to carve out a line about 1/4″ (6mm) deep and wide. Now tap along this groove in the same way as when splitting along the grain.

You may have to hit quite a bit harder than when splitting the slate in other ways, and the break will probably occur not at right angles to the end of the block, but slightly off it.

In commercial cut work, slate is sawn along this line rather than split with hammer and chisel, to achieve a more exact result, but for general walling or building purposes it doesn’t matter if the cut is a bit off.

Granite

The following tools are suggested for use on granite:

1. A 15″ (375mm) grinding chisel with a three-quarters square shaft with a flat top for striking, tapering to a 3/4″ (18mm) wide arrowhead grinding surface. A star drill may be used instead.
   
   Drills may also have waisted shanks, which are particularly useful on wider drills to prevent them jamming in the holes.
   For drilling a hole in which to set a metal post or similar, it’s best to have several drills, graduated in length so that as the hole deepens you can use progressively longer drills. This is easier than using a long drill from the start. Drills for this purpose should be 1/4″ (6mm) wider than the post to be set in, so that there is space around the post to pour in lead or cement for fixing. Various electric drills, including rechargeable portable drills are also available, and make the work very much quicker and easier.
2. Cleaning tool to remove rock dust when drilling. This is a light metal rod, 6-7″ (150-175mm) long and about 3/16″ inch (5mm) in diameter, with the last 1/2″ (12mm) flattened and curved as shown.
   
   You can also use a bicycle pump to blow out debris.
3. Six sets of shims and wedges. Each set consists of a pair of metal shims or ‘feathers’, narrow at the top and flaring outward so that you can grip them, and a metal wedge.
4. Lump and sledge hammers.
5. Steel wedges, 6″ (150mm), of the sort used in tree felling.
6. Crowbar and wrecking bar.

To split granite:

1. Drill a series of holes along the line to be split. Their distance apart is set mainly by the length of stone to be split with the available shims and wedges. Holes can be spaced up to 1′ (300mm) or more apart, but the closer they are the more accurate the results.
   To drill each hole, hold the grinding chisel or star drill in one hand, with the shaft perpendicular to the rock. Rest the drill point on the rock and hit the drill smartly with the lump hammer. Do not use very heavy blows, however, since these will not necessarily speed the work and, if attempted on other types of rock, may shatter the stone. Then turn the drill 60˚. Continue hitting and rotating the drill until the hole is 4-5″ (200-250mm) deep, or less for a small slab. Flick the dust out of the hole from time to time with the cleaning tool. Drilling granite by hand is a very slow, tedious job, so don’t exhaust yourself by trying to rush it.
2. When all the holes are drilled, insert a pair of shims in each with a wedge between. Pound each wedge once, moving down the line in consecutive order. Return to the first wedge and repeat the operation until a thin crack forms between the wedges.
3. Insert one or more large steel wedges into the crack. Retrieve the shims and small wedges. Then hammer the big wedges a few times until the split is wide enough to insert the crowbar or wrecking bar. Remove these wedges and lever the slab apart.

Schist and gneiss

Schist has definite planes of foliation and along these it splits much more readily than granite. Mica schist is the easiest type to split. Gneiss can be split using the same tools and methods but the results are less easy and predictable. Extremely irregular and tenacious gneiss may require drilling before it is split.

To split small slabs of these rocks all you need is a lump hammer and a cold chisel. To split and trim big slabs you need:

1. Two heavy sledge hammers, one square peined and one with one square and one bevelled pein. The latter should be as heavy as possible, up to about 25lb (11.3kg).
2. Lump hammer
3. Hand hammer with one square and one bevelled pein, about the same size and weight as the lump hammer.
4. Thin steel wedge. An old discarded axe blade is ideal.
5. Crowbar and wrecking bar.

It is easiest if two people work together:

1. Prop the slab up on edge using wooden blocks or stones.
2. Mark out a straight line parallel to the grain of foliation.
3. While one person holds the bevel-pein sledgehammer with its edge along the line to be split, the other pounds it with the square-pein sledge. Starting at one end of the slab, move along it striking one solid blow in each position. Repeat the sequence until a crack shows along the line which has been marked out.
4. Tap the old axe blade into the crack until it can stand without being held. Then drive it in farther with the sledge.
5. When the crack is wide enough, insert the end of the crowbar or wrecking bar and prise apart the slab.

To trim schist to a more precise shape:

1. Prop up the slab and wedge it in position with stones.
2. Hold the small bevel-pein hammer against the bottom of the surface to be cut away, with the head angled downward about 30˚.
3. Hit the bevelled hammer with the lump hammer to chip away at the face of the slab. Remove the face bit by bit, gradually moving the bevelled hammer higher on the face as the lower part is cut back sufficiently.
   
4. As you near the top of the slab, remove the props. Finish squaring off the top face by removing small chips of stone. To do this, hit the bevelled hammer while holding it perpendicular to the top face as shown.