The purpose of this page is to address some of the misconceptions surrounding the use of lime mortar and gallets or pinning stones. There is little guidance available to those wishing to carry out work on galleted masonry, a craft the skills for which have been lost over the centuries.
The following provides some hints on the correct execution of this type of work without setting out the process in detail.
It is essential to ensure compatibility between original mortar and new mortar introduced for repointing.
Non-hydraulic lime mortar and hydraulic lime mortar should not be used in conjunction with each other. Establish the form of the existing mortar and keep to the same in the new work. Cement mortar should not be introduced into historical buildings as it does not possess the correct properties. Also cement should not be added to lime mortar which is weakened by this.
The aggregate should comprise sharp sand graded to match that in the existing mortar. Mortar analysis is helpful in assessing the constituents of a mortar.
Mortar is designed to be weaker than the surrounding masonry so that it will decay in preference to the masonry. It is thus described as sacrificial, mortar being easier to replace than the more valuable masony.
Repointing with a strong mortar is a common fault that can have seriously damaging consequences. The picture illustrates a very typical repair carried out using a strong cement mortar, probably in the belief that the wall will be much more resistant to damage from the weather and physical damage. Unfortunately this is not the case as the mortar has failed to protect the brickwork. The strong mortar has survived and the bricks that were originally full
of character have eroded. It can be seen here that a modern replacement brick is unable to achieve the same pleasant textures of the original.
There is a rule of thumb that gallets should always be inserted into mortar joints that exceed a finger's width. If an existing wall already contains gallets, or there is evidence that they were originally used, they should appear in the new work matching the existing or introduced using other galleting in the area as a guide.
Gallets should be formed out of materials appropriate to the locality. The material will usually dictate the form of each gallet, flints being wedge shaped, slate small flat peices and iron stone depends upon its natural shape.
Gallets are sometimes described as wedges. As a description this is not incorrect but the word "wedge" is frequently misinterpreted. These days wedge is a term that usually indicates physical contact between the faces of the wedge and the adjoining objects. This would imply contact between the gallet and the adjoining stonework.But contact should be avoided as this can cause bruising to the stones and may lead to high stresses that result in flaking of the corners or arrises of the stones.
It is better to think of gallets as likened to the passengers on a crowded train. Although the people are packed or wedged into the train they do not have to be in physical contact to be unable to move or fall over. Each passenger is surrounded by some air. Similarly every gallet should be surrounded by some mortar.
The gallets should be sufficiently packed into the mortar joint to minimise their ability to move out of possition. Correctly inserted they will make a significant contribution to the structure of the wall.
SUMMARY OF TOP TIPS
1. Remember the rule of thumb; mortar joints wider than a fingers width should be galletted.
2. Use mortar that is compatible with the existing.
3. Salvage existing gallets for reuse, if appropriate.
4. Rake out existing mortar to adequate depth to accomodate the gallets, often up to 50 mm.
5. Form and insert gallets to match the existing style or in accordance with local tradition.
6. Use only the amount of pressure necessary to insert and position the gallets without disturbing the masonry.
7. Gallets should not be forced into joints between blocks of masonry; there should be mortar surrounding each gallet.
8. Protect new work from extremes of weather, hot sun, cold weather and driving rain using hessian.
The above is intended as a helpful guide to those who may be considering undertaking work of this type and is based upon our current level of knowledge and understanding. The information is provided in good faith but it is the responsibilty of the user to ensure that it is appropriate and applicable to any given project.