Joinery Page 2 - Notes

Or 'Idle Thoughts of an Idle Joiner'

Brief History of Sash Windows

Some years ago The Guardian printed my answer to a question in "Notes & Queries", as follows:

AN INTERESTING FOOTNOTE is the image of HMS Victory on page one, linked here in case you have not already seen it.
Were it not for this ship we might have been invaded by Napoleon and forced to have french windows! Details of the part played by the sash-windows in the Battle of Trafalgar are not recorded.

Accounts of the building of this ship are fascinating - the whole approach was meticulous with very tight control of design, specification and manufacture throughout, down to the weight of thread and stitches-per-inch of every part of the sails. Even the design of the windows in the stern was considered at length and generated enormous amounts of paperwork - a serious failure even here could jeopardise the effectiveness of the ship. I think it is very likely that the design of 'on-shore' joinery would have benefited from this process.


coloured glass Window at The Chapel, Main Street, Middleton
This narrow decorative margin is a common 19C fashion in windows and glazed doors This is acid-etched flash glass.
Flash glass is traditionally made clear glass to which has been added a thin layer of coloured glass during the manufacturing process. The resulting colour is somehow richer and brighter than glass of one colour throughout. The thin layer of coloured glass can then be removed by grinding, cutting, acid-etching or sand-blasting, to make a pattern, either a clear design on a coloured background or vice-versa as in this example, also from the Chapel. round window
Acid-etching is achieved by painting, printing or stencilling a pattern onto the glass in a suitably resistant material. The glass is then washed with hydro-fluoric acid which removes the uncovered parts of the coloured layer.
A technique also seen often is grinding and polishing to form shiny brilliant cut patterns. This is often seen in the square corner of margin glazed doors and windows and the square piece itself is usually known as a brilliant.

Old glass is usually very thin by modern standards. Even large pieces in Victorian windows 3ft square or more may be only 3mm thick. It is often highly polished - especially crown glass - fired in both sides. Also it may have been hand-polished by window cleaners several hundred times.
Some pieces of crown glass vary in thickness, from 5mm to 1mm. Smaller panes may be even thinner. Glass like this was always fitted thick edge downwards, which probably gave rise to the myth that glass creeps and sags with age.
Other pieces can be nearly perfect and very beautiful - thin and delicate with a highly polished surface and just the faintest ripple to indicate its origins - each pane a piece of skilled craftsmanship in its own right.
Old glass is often curved in which case it is fitted with the convex face outwards and deepest curve on the vertical axis. This gives a deeper putty fillet on the bottom edge which is better for weathering. When all the panes in a window are like this the result can be that familiar Dickensian christmas card effect of reflection and shadow.

Tragically, millions of tons of beautiful hand-made glass has been destroyed and dumped in recent years mainly due to the depredations of the plastic window trade

More notes on re-using old glass below

Why Double Glazing is a waste of money

As a rule of thumb the average house loses 10% of its heat through the windows and Double Glazing will halve this to 5%. If the heating component of your fuel bill is say £500 per year then Double Glazing will save 5% of this, i.e. £25 per year. Double Glazing your home is likely to cost from £2000 to £5000 so there is no possibility of it being worthwhile as a money saver.

The position is made worse by the fact that Double Glazing units often fail within 20 years or so and then need replacing. Even if new windows are being fitted anyway, the extra cost of Double Glazing is not likely to save money.
Fitting Double Glazing to existing windows is even less viable, since in order to make the Double Glazing units fit an existing frame they have to be made with a smaller air gap which greatly reduces their insulation effectiveness.
If the existing windows are sashes then the increased weight of glass creates a sash weight problem - there may not be enough room for the larger weights required, which are an expensive item to replace anyway.

So Double Glazing is usually a waste of money however you look at it.

Details vary - there are some circumstances where Double Glazing will save money, e.g. where there is high exposure to wind - tall buildings, high altitudes etc. You can verify the details yourself with one of the many DIY reference books, or a competent heating-engineer or central-heating installer is able to calculate accurate figures for a particular building - but don't ask a Double Glazing sales-person!

Alternatives to Double Glazing

The good news is that there are plenty of other ways to reduce heat loss which are also likely to save money. The simplest and most cost effective is to turn down the thermostat by a degree or more and to turn off radiators when rooms are un-used. Other methods involve expenditure which will eventually be recouped after a number of years.

These are– If all these things are done then the proportion of heat lost through the windows will be much higher, but the saving due to Double Glazing will be a bigger percentage of a smaller heating bill, i.e. the same - still not worth doing.

But isn't saving energy by double glazing still a benefit to the environment even if it saves no money?
The answer is almost always NO. The energy consumed in marketing, making, fitting etc. is much greater than any saving.

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Some hazards of home improvements

You don't know what you've got 'till it's gone
They paved paradise and put up a parking lot.

Joni Mitchell

People buy picturesque farm houses and cottages and then set about stripping, scrubbing, demolishing and replacing until they have finally eliminated all the quality and character which attracted them in the first place.
Demolition is easy. Prising off loose plaster or render is strangely satisfying and gives the illusion of rapid and thorough progress. Lime mortared walls can be knocked down in no time and a fit youth with a sledgehammer can reduce a house to rubble in a day or so. The temptation is to try and remedy the dilapidations of many years in one thorough purge.
This urge should be resisted. Building maintenance is a continuous process and the character of old buildings is often the result of minor alterations, repairs and additions done over many years. The enthusiastic new owners part in this process should also be minor, with an emphasis on repair, doing as little as possible.
Poke it with a stick – if it doesn't fall down then it's probably OK and better left alone.

Avoid destructive processes

Caustic-soda dipping

People regularly bring in to our workshop what appear to be a bundles of salt encrusted driftwood and ask for it to be put back together as it was – e.g.a welsh–dresser etc. Unfortunately once a piece of woodwork has been soaked in a caustic–soda bath it is ruined.

In this way, every year, thousands of tons of excellent furniture and joinery are wrecked forever and the patient careful work of generations of makers, repairers, polishers and painters comes to a dreary end.

External joinery is particularly unsuited to this process as most of it was intended for painting from new. This means that the material although quite good enough for painting; when stripped reveals big knots, filled cracks and holes, and water stains.

If you find that you have accidently aquired a stripped item then the best thing to do is to paint the poor wretched thing immediately.
This may not be easy as another trick of the stripping trade is to apply thick layers of an oily and dirty coloured wax "antique polish", adding insult to injury. This is probably made of re–cycled old chip–fat.
Unfortunately this takes very well to the softened surface and is difficult to paint without further stripping. If so then I would use " Nitromors" or similar. There are many brands of strippers which do not harm the wood beneath, unlike caustic soda.
A useful tip with these sorts of strippers is to clean off by soaking it up with lots of sawdust which can be worked in with a stiff plastic brush and then brushed off and swept or vacuumed up.

Caustic soda solution is very useful in one respect. We keep a bucket half full in a corner of the workshop into which we throw items of hardware reclaimed from old joinery. Brass, iron and steel clean up beautifully but some other metals are reactive and should be tested first. This is especially useful for removing paint from cast–iron hinges or sash–pulleys which would otherwise be un–useable

Sand blasting

I recently visited an interesting old house in Derbyshire where the owners had employed an enthusiastic sand-blaster. The result was that all the woodwork, including beautiful Georgian internal shutters and panelling, had lost not only its paint but also it's surface and moulding details and looked like rough suede leather. The soft sandstone fireplaces and other masonry had had the erosion equivalent of 1000 years in the Utah Desert applied in one afternoon, and some of the window glass, including pieces of un–replaceable crown glass, was now opaque!
The owners were obviously a bit shaken but they were trying to put a brave face on it and pretending that this was just what they wanted.
This sounds unbelievable but it is true and I have seen similar damage done in other buildings.
There is no place for sand–blasting in old building renovations, with the possible exception of cast iron, when decorative details have been lost under paint and tar, or rare circumstances where soft material is removed from much harder material. Even then care must be taken so that soft materials are not destroyed.
Unless of course, you want opaque windows – sand–blasted glass is more attractive than modern obscure–glass patterns.

Knocking off plaster and render and pointing

Poke it with a stick – if it doesn't fall off then it's probably OK and better left alone.
Where it is seriously decrepit it may have to go, but if much force is required then it is not necessary to remove it, which should be self-evident.
Inspite of this enthusiastic renovators chip away with great effort only to replace the lime mortar pointing with modern cement mortar – inferior in appearance, durability and performance to the lime mortar removed.
Even worse is the process whereby chipping out from sandstone masonry also chips away the arrises, giving the stones a rounded-off appearance like loaves of brown bread.
Render is a popular target. People seem to think that it is always a later cheapskate addition and so to be removed. Render in various forms is an ancient building technique, and many buildings were built to be rendered from new. When removed the masonry underneath may well be irregular, including rubble and rubbish which was intended to be covered.
When this is the case then a clue can often be seen in the masonry of quoins, lintels, cills and jambs; if these are proud of the masonry infill then it is probable that the infill was rendered flush with the masonry. This can be very attractive when the masonry is painted a different colour from the render.

Floor Sanding

The risk here is that getting down to the depth of the worst scratches or stains may mean taking off 6mm or more which is a lot to remove from boards which are only 22mm thick, but some floors can take it. Woodworm is a problem in that they tunnel along just under the surface until they exit by taking a sharp turn to the top. Taking off the surface can reveal a spongy network of tunnels which would have been better left unexposed.
In any case sanding makes the foor look horribly new, with no patina of wear and tear. An alternative to be considered is careful washing with soapy water or sugar soap. If very dirty then a floor scrubbing machine is a good idea. The result can be pleasing, and when it is very dry colour can be restored by treating with half&half raw linseed oil and real turpentine.

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Glass replacement

I always re-use old glass if available - reclaimed from earlier jobs or retrieved from skips and other window suppliers. It is possible to buy new crown glass but it is expensive. A cheap alternative is horticultural glass sometimes sold as 'dutch lights' which has ripples and other imperfections resembling old glass.

Renovating frames

Glass removal and re-glazing is much easier on a bench in a workshop rather than on site. A better job and better value is achieved even allowing for temporary boarding up and other complications.

It is very difficult to remove glass from an old window without damaging either the glass or the frame as the putty is usually rock-hard and scraping and chipping can take all day and need the skill of a dentist, but if only one pane in a six or eight paned light needs replacing then there is no other way.

If the frame is to be scrapped then it can be separated from the glass by sawing through all the corners and joints. The glass is then held in one hand and the wood is gently tapped away with a hammer.

If the glass is to be scrapped - cut it with a glass cutter, drape a cloth over it to avoid flying splinters and then knock it out. I find it useful to cut round leaving about an inch of glass still in place after knocking out the middle, and then to apply a hot air gun to the glass and putty. The glass can then be pulled away using grozing pliers , bringing most of the putty with it. This is much easier then trying to remove putty on its own, the glass seems to conduct heat into the putty fillet softening it from within.

Dismantling frames

Sometimes both frame and glass are to be saved e.g. when joints have loosened or when only one component needs replacing (usually the bottom rail) then it is easier to pull the frame apart and separate all the components rather than trying to remove the glass.

Many frames (not all) are dowelled through the joints, sometimes only at the meeting rail where a properly made wedged joint is not possible.
Dowels are usually square pegs split with an axe, knocked into round holes. This is not obvious as the dowel gets compressed and rounded as it is hammered in so it will not look square.
However it will be tapered, having got progressively compressed as it is forced through. This means that it must be removed in the opposite direction i.e. a drift hammered into the smaller end of the hole.
If it won't knock out then it must be drilled.

Why square pegs in round holes?
Splitting follows the grain so a split dowel is stronger than one sawn, turned or whittled. A round hole is easy to drill but a round dowel is quite difficult to make, and not necessary.

The rest of the frame can usually be knocked apart. Heat is applied to the ends of tenons may soften glue, and the tenons of glazing bars can be knocked through with hardwood drifts of the same section.

Re-using glass

The problem with re-using glass is that it may not fit back into the renovated frame as the joints may have been cramped up tighter and measurements changed. There is little margin for error as old windows have small glazing rebates. Also the edges are often chipped, scratched and encrusted with hard putty.
The simplest thing to do then is to cut off the messy edges and to save the glass for the next job as it is now too small to fit, and instead to use glass set aside from previous jobs.

Old glass is rumoured to be brittle and hard to cut but I have never found this to be true. Cutting is no problem as long as the glass is clean and the cutter is newish. Cutters should be discarded at the first hint of bluntness - if you wait too long then you will eventually spoil a piece of glass worth more than the cutter.

Old glass may have chips and spalls at the edges - usually where sprigs were used or where it was nibbled off. These can suddenly turn into cracks as the glass is moved about after removal. This is especially true of large pieces of shop window glass. The weight creates stresses and large pieces can suddenly shatter, particularly when they are being carried. This can be dangerous.

If the glass is so special that it must be re-used on the same job then it may be necessary to re-build or make a new frame around the glass measurements - sometimes no two panes are the same. Old windows have very small glazing rebates - often 14mm x 5mm, sometimes smaller. This means that there is no room for sprigs which in any case should never be used except as a temporary fixing e.g. of glass fitted on site.

Putty, Sprigs and wetting off

Sprigs should not be used. They are a common cause of glass breakage as any movement in the frame will increase pressure at the sprig. Also putty often cracks around them, letting water in. Lights with small panes of thin glass (2 or 3mm) without sprigs can be re-hung fairly promptly. Heavier glass might fall out under its own weight and should be left longer, but 3/4 days is usually enough.

Putty setting is speeded up by wetting off - spraying the re-puttied window with water or flicking it on with a paint brush. This also makes subsequent cleaning much easier - putty smears can set very firmly if not wetted off promptly.
The only problem I have had with un-sprigged glass and fresh putty was on a very hot day near a busy hill - fresh putty is slightly thixotropic and the combination of heat and traffic vibrations caused it to run and sag.

Putty should always be fresh, not older than 2 or 3 weeks. Look for the date mark on the lid. If there isn't one then don't buy it.
It is best bought from a glass or builders merchant where rapid turnover means freshness.
It is worst bought from a DIY superstore where it may have been on the shelf for months.
When newly opened there should be no trace of oil and even less should there be any sort of skin or setting. It should be possible to stick your hand straight in and pull some out cleanly without it sticking. If not perfect then throw it away (or take it back to the shop) - it's not expensive.

Rebates should be recently primed and the glass should be clean and dust free. The glass is pressed into a thin bed of back putty - but not too thin, say 2mm minimum.
I find it easier to complete one fillet at a time, i.e. to finish one edge perfectly before starting another - starting the next without touching the one before and not going back at all. Also I always start on the most difficult edge, furthest away or needing left hand etc. so that it gets easier as you go. I've tried various tools and have settled on a 1.5 inch stainless steel paint scraper for preference. I keep it clean with steel wool and the edge and corners are very slightly rounded off and so don't scratch.

The putty fillet should not extend beyond the rebate - the two sides of the rebate and the face of the putty should make a neat triangle in section, with a clear line of sight through.


Weights for the top sash should be slightly heavier than the sash itself, say 10% maximum, so that the sash will stay up when not fastened., and vice-versa for the bottom sash i.e. 10% lighter. More here soon
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The Society for the Protection of Ancient Buildings (SPAB) site, with links to many other sites, is at

The Regional Furniture Society has a new web-site at


We have closed our workshop at Via Gellia Mill and temporarily ceased trading pending the opening of our new workshop in Middleton. This may take some time as extensive alterations and sound-reducing measures must be in place before we can re-open.
In the mean time I will be happy to continue trying to answer questions on traditional joinery and also to discuss future work projects and can be contacted as follows;

Jacob Butler
The Chapel,
Main Street,

Telephone 01629 822170
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