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Grave Concerns : David Francis

20 June 2016
Broken dowel

A broken slate dowel. Fixing has always been a problem.

David Francis is a hands-on mason who has specialised for many years on the memorial side of the stone industry. He was Technical Advisor to the National Association of Memorial Masons, writing manuals and City & Guilds Qualifications. If you have an issue regarding any aspect of memorial masonry, David is happy to help. Send your questions or comments to David at [email protected]

I was looking round a churchyard the other day with one of the church wardens when we came across a marble cross that had broken between the cross and the first base. This break revealed a slate dowel.

The church warden asked me if it could be repaired and if the slate needed to be removed.

The slate seemed to be in good condition, so my advice was to tell the mason who would be refixing it to drill through the slate until the white dust of the marble was evident, drill on a bit deeper to make sure the dowel was into the marble and then cut a stainless steel dowel of the appropriate length.

The joint between a cross or statue is inevitably a weak part of the construction. Over the years many materials have been used for this joint – brass, bronze, aluminium, wood, even plastic ballpoint pens in a few unscrupulous cases.

The oldest type of joint is a joggle, where a projection from one piece of stone or timber fits into a notch in another to prevent lateral movement. An example of this can be seen at the top of some of the stones at Stonehenge.

When a joggle is used in the joint between a cross and its base in  a memorial it sometimes sheers off. The main reason for this is the amount of work involved in shaping the stone to make the protruding part of the joint. The chisel work in forming a shoulder introduces stress into the material.

What is even worse is the cutting, which can introduce heat to the joggle, imparting stress where it is least wanted. Heat into marble and granite, if in a restricted area, is bad for the material.

The whole idea of adding a dowel is to make a joint stronger by creating a physical joint or support between pieces of adjacent material.

The problem that fixers find when they are renovating memorials with such joints is that, over time, erosion of the material used for the dowel or the adhesive used to secure it in place has failed.

The fixing is then no longer doing its job of keeping the construction safe. Iron rusts and can break the shaft of the cross and often thin or soft metal dowels give way if the memorial goes out of level.

Today it is recommended that not only stainless steel is used but a second dowel to help stop any twisting in case the joint should become loose.

A second dowel is not always possible. In my scenario at the start of this column, for example, the shaft of the cross was less than 100mm square. That means even a 12mm dowel seems too big, because, as with any renovation work, the methods used must not compromise the authentic look of the memorial being repaired or present a threat of potential damage to the original material.

Many years ago my business produced and fitted a number of shop fronts. The company we used to supply the fixings for this work was also asked to specify the sizes of dowels for marble and stone for our monumental business. It supplied us with 10mm dowels of various lengths – and they worked well. I could never understand how the currently recommended dowel sizes came about.

The memorial I mentioned at the start is marble. Marble is not usually approved in churchyards now, but existing memorials will be repaired.

Not much of the shaft of a cross can stand the stress of a large dowel hole. Thinner sizes do work and it is the length of the dowel that is most important.

 

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