Tuesday, 5 July 2011

Acid Etching a Door Panel

Acid etched panels, mainly for doors, became popular after the general availability of hydrofluoric acid in the mid 19th century. These were done on plate glass of 6 mm or 8 mm with a process known as bite and grind in Glasgow. This was sometimes called French embossing in other localities. A number of these windows have survived both fashion - where they were removed for leaded glass replacements - and wear and tear.

The design skills in producing these door and window panels, often with classical or oriental influences, were of a high order. However, as none are signed they have not attracted the interest of art historians the way stained and leaded glass has. They are also monochrome, so the immediate appeal of colour is not available. However they present a number of subtle interactions with the external environment, in addition to providing both light and privacy.

Through breakage, these panels have become more rare and when broken are being replaced with sandblasted versions rather than using the original techniques. This is due to the ease of producing computer generated graphics onto vinyl that is cut with plotters. It also reflects the reduction in the number of studios willing to make use of hydrofluoric acid, and everyone's concern with costs.

Reasons for Traditional Process
So why etch and grind these windows to reproduce the original techniques and designs? One long term reason is that it gives a better surface as oils are not absorbed into the surface. The sandblasted panels show finger prints quickly and are difficult to clean, even when blasted with fine sand. It appears the force of the sand hitting the glass causes microscopic fractures in the surface of the glass, allowing it to hold the dust and dirt of everyday living.

The colour of a ground panel is not as white or opaque as sandblasted surfaces and so allows an interaction with the colours of the external environment. The sandblasted surface provides an opaque ground that is not affected much by the external colours.

The detail that can be achieved - by using a combination of vinyl and bitumen to act as resists to the acid - is much greater than with sandblasting, as the resist does not have to cope with the high pressures of sand hitting the resist and glass. These high pressures (around 120 lbs. per square inch) cause small pieces of resist to be blown off and the detail is lost.

Importantly, these sandblasted windows are not authentic. The clear parts of a sandblasted window are raised and the frosted parts are eroded away by the blasting process. In an acid etched window the clear parts are lower than the general surface of the window. Also, in the sandblasted window the clear parts are optically clear and can be seen through. In the acid etched window the "clear" parts are uneven and so cannot be seen through, providing additional privacy. These areas provide a sparkle to the window when seen from a distance, rather than the flat reflective surface of the clear in a sandblasted window.

The example being used in this note about the process of acid etching is approximately 120 years old. It was broken by an impact from an object thrown from the street.

In the days before float glass, plate glass - variously 6 mm or 8 mm - had to be ground flat, as the large sheets at that time were drawn and so had imperfections in thickness. For mirrors this was especially important. It is likely that the process 120 years ago was to use plate glass that was not yet at the polishing stage. The bitumen would be applied to the already ground surface and etched. When cleaned, it was finished.

Nowadays with vinyl it is possible to do really crisp geometric lines. But vinyl needs a very smooth surface to adhere tightly. Thus the process is reversed. etch first, grind second. This is the process that will be described here.

The panel arrived in the state illustrated below. Some of the damage at the upper parts of the panel were done by the glaziers who removed it.

Panel roughly cleaned and partially assembled

The first process was to remove the bits of putty, clean and then assemble the pieces and take rubbings for a record and to provide a backup to the glass if anything goes wrong. The rubbings are taken by taping the paper firmly and stretched around the glass. You then use a piece of encaustic wax to rub over the whole panel - or at least the areas where you know the design is. This provides a coloured ground in which the clear etched areas, which are lower than the general surface of the glass, are white. This process is the same as taking a brass rubbing.

General View

This rubbing was taken on tracing paper to make it easier to see where the etched areas were and ensure all the detail was taken in the rubbing. Cartridge paper would have provided a more stable material for preservation of the rubbing, but the choice is up to the worker.

The following two pictures illustrate the level of detail achieved in the glass by acid etching, and that the detail is below the general surface similar to brass plaques. The impact point can be seen in the rubbing of the lower part of the window.

Central Detail

Bottom Detail

As this is for a door, laminated glass was chosen as the material to etch. You could use toughened glass, but it is a safety rather than an security glass. Security glass is required for the door because although one side of the glass may be broken the plastic interlayer keeps the window whole, making breaking through very difficult. Thus laminated glass is the most common choice for acid etched door panels. It also meets the British Standard requirements for glazing of entryways.

When you receive the glass cut to size, or if you do it yourself, you need to insure there are no sharp edges to avoid cuts in handling and during acid etching. This can be done by running a diamond pad or any other grinding block along the edges to smooth them. Pay special attention to the corners, which should be slightly rounded. The smoothness of the edges helps to prevent holes in the acid bath or in the gloves you use to protect yourself from the acid so this is an important part of the process.

Vinyl Application
Cover the bottom of the glass with a clear vinyl to prevent scratches. Use clear vinyl to be able to see through to the glass you are copying from. For the same reason you need to use clear vinyl on the top of the glass. If you are creating a new panel you can use white resist, as it is easier to see the design as it is cut from the resist.

Application of the vinyl is done in the same manner as people who apply vinyl transfers to vehicles. A spray bottle is filled with a solution of 10 - 20 drops of washing up soap for every litre of water. The proportions are not critical but avoid too much soap, as a foamy film will be created. The soapy water allows the bubbles to be worked out from between the vinyl and glass.

Apply masking tape to edge of the vinyl with half of it overhanging. Align the edge of the vinyl to the edge of the glass and smooth the tape onto the glass glass edge and around to the bottom. So you now have the vinyl and its backing squarely aligned to the glass. Spray the glass with a film of soapy water. Peal some of the backing off the vinyl and begin to smooth the vinyl down with a tiling or screen printing squeegee. Or failing that you can use a credit card - although that will take much longer. Any non-metal thing with a firm smooth edge that has no sharp corners will do, but rubber and plastic seem to work best.

Smooth the vinyl out as you pull the backing off. If you have areas where there are bubbles trapped, you can lift the vinyl - it is not stuck firmly yet - back to the bubble and smooth it out again. Be careful to avoid pulling hard at the vinyl as it will stretch, leaving you with bubbles impossible to remove - and distortions in any pattern that has been cut prior to attaching the vinyl to the glass. As you smooth the vinyl in the direction of the newly laid vinyl, the soapy water will be pressed out from under.

Apply the vinyl on both the bottom and top in this way. You then need to leave the whole alone for several hours, or better, overnight to get the full adhesion of the vinyl to the glass.

Providing the Detail
The geometric details are cut through vinyl. Very sharp blades required for cutting vinyl, just as for sandblasting. A scalpel or craft knife with interchangeable blades is suitable. The sharpness is to make cutting through the vinyl easy. When or if the blade does not cut through the vinyl easily, it is time to change the blades. The blade will have a little resistance from the vinyl until it slips down to the glass where it will slide smoothly. No hard pressure is required nor should be applied. The blades should come to a fine straight point. This is to allow turning at acute angles without lifting the blade.

To assist in cutting straight lines, a thin metal ruler of appropriate length is important. Plastic or wooden straight edges can be cut into easily by the sharp blade, so giving a wavy line. Stability in holding the metal ruler can be assisted with a few pieces of blutack stuck to the back. This will quickly become dirty, but provides the essential friction to keep the ruler aligned while cutting the resist.

If you have difficulty in seeing where you have cut the vinyl, you can spread a little powdered poster paint over the vinyl. This will be held in the cut vinyl enabling you to see the cuts you have made.

The areas of non geometric detail have the vinyl cut away and painted with bitumen. The bitumen is painted up to one side of the line or area to be etched. Then the bitumen is painted to the other side of the line. Finally, all the areas to be kept from the acid are painted in. The whole process is similar to painting in negative.

Note all bitumen work in this example was done by Brian James Waugh.

General View of Panel Ready for Acid

Central Detail

Bottom Detail

You can see from the detail pictures that the bitumen has to be built up to be a solid black to resist the action of the acid. The advantage of the bitumen is that you can get a graduated etch by applying the bitumen more thinly, or by careful application of white spirit.

Acid Bath
Select a stiff board at least 300 mm larger than the panel. Nail 25-50 mm thick battens in a swastika style around the board. You must leave at least 100 mm space each side of the panel. Leave one batten loose at the narrow end to allow the acid to be poured away at the end of the etching. This end board can be clamped in place for safety.

Check the set up for level. If necessary level up the board with wedges. Ensure there are no sharp elements anywhere inside the "bath". Then line the bath with heavy duty plastic in at least two layers. Lay the plastic over the battens ensuring there is generous overlap. Gently push the plastic into the container formed by the battens. Ensure there is a large overlap of plastic on the end to be opened to allow the plastic to be in the collecting container as you drain the bath.

Ensure again there are no sharp edges or points on the bottom board. Carefully place glass into bath. Before filling the bath with acid you need to do two things. First check again that all the design is complete and ready for etching. Second, get the feel of the thickness of the vinyl by moving a sharp wooden stick from the vinyl to the glass. This is important for testing the depth of the etch later while there is acid on top of the glass.

Now to emphasise the safety for yourself and others.

You need heavy duty extraction if you are doing this inside. If doing it outside you need to make sure neighbours and their pets cannot get access, also think about wild life access, as this process will take about two hours and you won't be standing beside it all the time.

Always use respiratory equipment that is suitable for acid vapours any time you are within two metres of the acid bath.

Always use long thick rubber gloves. Check on their effectiveness by immersing your gloved hands in a bucket of water for at least a minute - this will seem a long time. If there are any leaks you will feel the moisture either while in the water or afterwards. If in doubt get another pair of gloves and check them too.

A rubber apron and wellington boots are good added precautions. Use all of these whenever you go close to the open acid bath.

You can buy hydrofluoric acid in various qualities and strengths. The quality ranges from technical to analytical, and from 20% to 60% solutions. Choose the technical grade acid as it is cheaper and being about 98% pure is of sufficient quality. Choose a 40% solution. At 60% the acid fumes at room temperatures and so is doubly dangerous. At 40% you have a good combination of safety and ability to vary the concentration. In general the stronger the concentration the quicker and rougher the etch. Clean 10% acid provides a smooth, but slow etch. For this application, 20% is a good concentration for both reasonable speed and clarity.

Note that when diluting acid, you decide on the amount of diluting liquid - water in this case - and fill the container with that amount. Always add the acid to the water, never the reverse. Adding water to acid can produce an explosive reaction and with the dangers of this acid you do not want splashes of even the smallest amount. Taking the acid from 40% to 20% requires equal amounts of water and acid. Put 500 cc water into a large measuring jug - photographic suppliers have appropriate ones. Then slowly add the acid to bring the level up to 1 litre. This gives a 20% concentration. Now cap the concentrated acid. Then pour the diluted acid into another wide mouthed high impact plastic bottle if you are not going to use it immediately. Keep all acid under lock and key except while you are actively using it. You, of course, have put all your safety gear on before doing any of this diluting.

Still with your safety gear on, add the acid from a large mouthed plastic bottle slowly at one edge of bath. Avoid all splashing. Adjust the bath for level as required by use of wedges so the same amount of acid is covering the whole of the glass. Only enough acid to cover the glass by 2-3 mm is required.

Allow the acid to work at least half an hour. The time is dependent on the strength of the acid of course, and on how deep you want the etch to be. Check on depth of the etch on the
vinyl portions. Never check on the bitumen, as it can be damaged easily after being in the acid for a while. The etch is definitely deep enough when a bloom of white appears on the areas exposed to the acid. This white is the residue produced by the action of the acid on the glass.

Acid bath ready for adding the acid.

Emptying the bath
When the desired degree of etching is achieved, you gather the receiving trays, funnels, and bottles and ensure the receiving container is supported just below the level of the bath to avoid splashes being caused. A large photographic tray is an ideal initial receiving container. Arrange the plastic sheeting so that it all is within the the container. While holding the edges of the plastic up, un-clamp and remove the loose batten by sliding it out from the side. The acid will begin to slowly drain into the container. As the flow reduces to very little, you can increase the height of the opposite side. Allow the bath to drain for 2 - 3 minutes. Then carefully remove the large tray of acid and decant it into the bottles. This is often done by transferring the acid to smaller trays and from there by use of a funnel to the storage bottles.

Rinsing everything that has been in touch of the acid is now the priority. Access to a hose is a benefit as lots of water needs to be used to assure yourself that there really is no acid left. A final check can be made by use of a solution of bicarbonate of soda. When this is applied to any acid, a fizzing reaction is visible. If you observe the fizz, you need to use more water to further clean the panel or implements. When there is no more reaction to the bicarbonate of soda, you can take off your safety gear.

Panel out of Bath and Rinsed with the Resist Still on the Glass

The powdered residue in the etched areas will be difficult to remove at this stage when the panel is still damp, but when the panel is dry it is easy to remove - use respiratory equipment during this part of the cleaning.

Vinyl Removed

For safety, I spray a little water over the whole panel to dampen the powdered glass. It is easy then to brush it up and put it into a plastic bag so it does not harm the dust bin men or get into the studio atmosphere. Now it is possible to work without respiratory equipment. Peal the vinyl off the glass carefully. It is likely to be strongly attached to the glass. Try to pull off small areas at a time. If you need to tackle large areas try to pull the vinyl as close as possible to parallel with and near the surface of the glass.

Cleaning the Bitumen

Bitumen is cleaned with white spirit and a brush. This is very messy and bitumen colours your skin and clothing, so gloves and old clothing and aprons are a good idea unless you want to throw away your clothes and spend a week trying to clean the bitumen from around your nails. The white spirit takes the bitumen off in just a few minutes. Lots of paper towels or rags are required to mop up the diluted bitumen. This is done in stages - get the worst off and largely mopped up. Then add more white spirit and rub off the remaining bitumen with more rags or paper. This will need to be repeated a couple of times. Really stubborn areas can be cleaned with a bit of white spirit and a small nylon brush.

Cleaned Panel on a Supporting Board

In this panel one line was missed out. This was discovered during the cleaning up process.
The picture illustrates the use of the vinyl as a small bath. The surface tension of the acid is enough to hold it within the area. However the edges of the vinyl were raised for added safety. Again when finished move the panel to the edge of the board and drain the acid into a container as for the whole panel. Rinse with lots of water and do the checks for residual acid.

Repair of Small Area

Then you are ready for grinding.

Respiration is required, good ventilation of the studio is necessary, and HEPA filters on vacuum sweepers are needed to avoid getting the dust from the sand and glass into the atmosphere of the studio. If you can do this outside, it would be better.

The grinding plates are simply pieces of glass that are larger than hand size with another piece of glass stuck to the top to act as a handle. New plates have the edges smoothed to avoid any scratching of the glass to be ground. The corners are also rounded for the same reason and to avoid any cuts.

Grinding Plates

In the example above, the smaller plate has been made from 6.4 mm laminated glass. Careful inspection shows that after five windows this plate has lost about half its thickness. So the plates do wear out.

Partially Ground Panel

In this photo you can see the grit filling the etched parts. This does no damage to the etched portions as there is no abrasion in the depressions. The grinding starts with a slightly coarser grit than the finish. This is for convenience. It would be possible to start with the 220 grit, but any imperfections in the glass are more quickly overcome with the 150 grit. It of course, does require a second grinding with the finer grit doing it this way.

Only moderate pressure is required in the grinding. Heavy pressure does not speed the grinding much and runs the risk of adding gouges to the area being ground.

The grinding motion is generally of a circular motion, although along the edges it is easiest to us long linear motions. The circular motions avoid straight line scratches in the glass, which are more difficult to get out than circular ones. The grit does break down as you grind, so add small amounts of new grit when ever the sound changes from the grinding, gritty sound or the grinding plate seems to be gliding over the glass from too much powder in the grit, as a car aquaplaning on too much water.

This shows how to check that the glass is properly ground all over. The panel appeared to be fully ground from directly above, but looking at the glass from an acute angle, you can see reflections showing the glass is not fully ground in those areas. So more work is required until there are no reflections and the ground areas have the same appearance.

This shows that even laminated float glass is not perfectly flat. This little "trough" appeared during the grinding. The use of 150 grit soon eliminated this, when 220 grit was making little impression.

Having completed the grinding at 150 grit, you now need to re-do it with 220 grit. How long do you need to grind to get the surface to 220 from 150 grit? You could use your judgement, but because you cannot see or feel the difference reliably, you need a witness. For my witness, I place lines from a paint marker randomly on the high areas of the glass - never in the acid etched parts. These do not need to be wide or heavy, just enough to be visible. When these lines have disappeared, the 220 grit has completely eliminated the marks from the 150 grit. It takes longer than you think, so you do need that evidence of when the process is complete.

The whole grinding process took about 3 hours of manual labour. Why not use power tools? Control, and the fear that the speed of power tools is such that gouges could be created are the main reasons for doing it by hand. And after all it is a hand crafted piece of work.

Panel at the studio door

This shows how the panel reacts to the density of the colour outside. It does not provide a uniform grey or white. It reacts to the dark frame of the studio door at the bottom with strong contrasts, while against the concrete roadway, there is little contrast. In the middle it reacts to the darker elements, and at the top it begins to react with the lightness of the sky.

Inverted Panel Shows the Detail at the Bottom

Note: All the bitumen work in this panel was done by Brian James Waugh


  1. Thank you for posting this embossing process. I am in the US and became interested in embossing back in 1980. I was so drawn to it, as if I'd done it in an earlier life, that I set off to London to learn all I could. That also included learning brilliant cutting as well. Eventually I also learned silver mirroring and gilding on decorated glass. I made a business of it which continues to this day.

  2. The designs etched out in glass doors are created by the finest designers keeping in mind the latest trends and the applicability of the user.