French Embossing and Acid Etching

This an article written in 1999 at the beginning of my discovery of acid etching. It may be useful to compare what I was doing then and what I recommend now - as well as the differences in style. It was written as a discovery log rather than a "how to" presentation.

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An Exploration of French Embossing

In a fit of enthusiasm, I agreed to take on the reproduction of a large, broken 1870s acid etched front door panel and promised that the work I did would be done in the French embossing manner. It later turned out that the owner had received promise of grant aid on condition of having the work done in a traditional manner. Since I was the only one to be foolish enough to promise traditional workmanship, we got the job. This was the beginning of several months of discovery.

What follows is a combination of the process of discovery and an explanation of methods. As it is a long document it is divided into sections:

Description of French Embossing

White Acid Substitutes

Grinding

Biting

The Bath

Sealing the Bath

Safety

Emptying the Bath

Cleaning and Finishing

Description

French embossing is generally the application of one or more surface treatments with acids to obscure the glass, followed by other treatment(s) to bring the design areas back to clear. The clear areas are not optically clear, but like lightly stippled glass, which lets a lot of light and colour through, although not actual shapes. In the past this has been done by the use of white acid for which there are many formulae. These involve hydrofluoric acid, ammonium bifluoride, sulphuric acid, sodium carbonate, hydrated lime, barium sulphate and sugar in various combinations and proportions, many of which are unstable, and none keep. One worker related to me the tale of a batch of white acid which had gone off, being rejuvenated by the addition of urine contributed by all the workers. Its effects are to some extent unpredictable. Still, it was what the client wanted.

White Acid Substitutes

The search for a supply of white acid proved to be unfruitful. Chemicals suppliers have no idea of what it might consist. Some studios which are reputed to use it would not reveal the recipe, or denied using it. One studio admitted to having used it a long time ago, with explosive results. As the search seemed to be fruitless, I went to a big commercial glass treatment company with the idea of getting them to etch the glass upon which we would put the resist. It was during this conversation that the person who was responsible for the surface treatments, queried whether I had ever considered bite and grind method of embossing. had to admit that I had not, as I didn’t know what bite and grind meant. He kindly explained, rather than throwing me out as an amateur. The bite is the acid etching of the glass and the grind is the use of a fine grit between the sheet and a plate of glass. An alternative method was found!

Grinding

Traditionally the grit used for the bite is emery powder. This is difficult to find. Enquiries brought the knowledge of aluminium oxides as a more consistent and long lasting modern alternative. Experimentation settled on a starting grit of 150 with a finishing 220 grit. This gives a very fine satin finish which is easy to clean. The grinding method is to make a glass grinding plate, put a little grit on the glass, and rub the plate over the glass to create the obscuration.

The grinding plate is made from a piece of 4 mm glass about six to nine inches square, with a stack about three pieces of glass cut to fit easily into the palm of the hand. These are stuck together with silicone or other glue. The grinding plate must not be small, as that leads to the possibility of nosing the plate and creating scratches, or forcing the grit into the etched areas. Larger plates are simply too much work, as they require much greater pressure at the centre point to be effective. I suppose machines could be used to do this, but we completed the window by hand grinding because it is relatively quick. As a guide, t1.2 square metres of glass was ground in an afternoon. It provides a much superior surface to even the lightest of sandblastings, as it is easily cleaned, smooth to the touch, and just as uniform as the worker wishes to make it.

Scratches are a constant danger. One caution about the changing of the grit is important. Be very sure to clean absolutely every trace of the coarser grit from the glass before using the finer grit. If you don’t, you will leave scratches from the coarser grit within the final finish, which will require a lot of additional grinding to eliminate. Secondly, scratches on the back of the glass can ruin the whole project. Do not slide the glass - ever. Lift and tip the glass, never slide it. Any trace grit will scratch the glass, and have to be polished out, or the sheet discarded. In general, the cleaner the atmosphere you can find for this kind of work, the more successful it will be.

Biting

The traditionally recommended sequence is to acid etch first, and grind second. Testing showed that there is a tendency to trap quantities of grit at the points of the etched design. As the grit builds up, it creates tiny, random scratches in the etched portions of the design, especially at pointed areas. This appears as a fine clouding of the surface, and if it could be controlled, would be very useful in creating depth and form. As it can't be controlled, the reversal of the traditional method was tried. The satin etch is so smooth, that it is possible to make the resist adhere closely enough to the glass to resist the action of the acid for about three hours. This is possible, but only if high tack adhesives are used on the resist. To get this, the help of a sign making firm was enlisted. The vinyl used to make signs for the sides of vehicles has a strong adhesive which has the required adhesion for the long acid immersion time required and can be supplied in sufficient widths for large projects.

The Bath

The opening for the glass in this front door was 760 mm by 1525 mm. This is much larger than any acid bath I have ever owned, even larger than my bath! The traditional technique is to place the sheet of glass to be treated on a bench with wedges underneath to level it up. Then malleable tallow is applied to the edges as a dam to hold the acid which is poured directly onto the glass. Malleable tallow is smelly stuff, and made from all sorts of difficult to render animal material like bone marrow. As BSE had, and still does effect us, malleable tallow is not allowed to be sold, even if it is produced. So a substitute for this had to be found. Plasticine was tried, but did not have even minimal protection against leaks. A beeswax and linseed oil mixture was considered, but rejected on the grounds of cost (beeswax being a very expensive commodity in the UK). Suddenly, a brain wave struck, paraffin wax (essentially candle wax) might work. It does! But it has to be applied in liquid form and therefore is hot. Being liquid, it can not be applied like malleable tallow to create a dam around the edge of the glass.

The solution to creating a combination dam and bath appeared to be the construction of a wooden frame with a rebate into which the glass can be placed. Tests showed this would work. The frame was constructed about 10 mm larger than the glass to be treated with a large (20 mm) rebate. At one corner a hole was drilled and a length of small diameter plastic tubing was inserted. A plug for this was created (although a plastic valve at the external end of the tube could be used). The internal edges of the wooden frame were first treated with a film of paraffin wax to protect the wood from the acid. This precaution is not strictly necessary as the action of the acid on the wood is relatively slow, but it is safer and more certain to produce a firm seal.

The glass, now covered with the vinyl resist, which was turned over the edges, is placed in the frame. You get the glass into the frame by getting help. One person holds the frame upright, resting on one long side while others take the two ends of the glass and tip it up to vertical. They then lift the glass into the bottom rebate of the frame and tip the rest of it into the other rebates of the frame. Everybody then helps to return the frame containing the glass to the horizontal. The frame is levelled up by use of wedges - two at each end and two on each side to maintain stability. A large spirit level is placed on the glass and by gently tapping the wedges, the glass is levelled. Pouring liquid wax at the edges of the glass without levelling would allow the wax to run to the lowest point without necessarily filling all the gaps, and it prepares the glass to receive the acid without further movement of the frame.

Sealing the Bath

Wax is flammable, so care and constant observation is necessary to avoid creating a fire while melting it. The safest melting method is to use a double boiler arrangement, so the wax can never go above the boiling point of the water (a very safe temperature for the wax). The wax was heated to its liquid state, allowed to cool and poured just before solidifying. This stage can be seen as a film beginning to form from the edges of the pan, like ice from the edge of a bucket. Wax that is too hot just flows through the gaps without sealing them. Repeated application of the wax in layers is necessary where there are sizeable gaps. It is a matter of judgement on how hot the wax is when pouring, especially in layering. The wax being added has to be hot enough to melt the surface of the wax already laid down, to make sure there are no fracture lines between the two applications. But if the wax is too hot, it melts through the previous layer(s) and creates a hole.

Aciding

It is also possible to bridge a gap from underneath by use of plasticine. The plasticine is worked and spread from the under surface of the glass to the wood frame, but not pushed between the glass and wood, where the wax is required. The wax is poured the over the bridge by the plasticine. Once the level of the wax has built up to the top surface of the glass and has sealed all the space between the wood and the glass, the conditions are created for the application of the acid. You could, of course, test the effectiveness of your sealing by putting water into this newly constructed bath, but you would then have to break the seal on the drain, tip the whole up to allow the water to run out and re-level the whole. If the wax isn’t dripping through, then the acid shouldn’t either. So check for wax drips throughout the sealing process.

Safety

Hydrofluoric acid is dangerous! This isn’t the place to discuss the safety procedures, but make sure they are followed at all times. The etch for this process is a long one, so make sure the area is very well ventilated. In the UK the acid comes in 40% to 52% solutions depending on the supplier. All of these are too strong to be used without dilution. Dilution to about 15% is appropriate. Remember, add the acid to the water, NOT the other way round. Ensure you have a large plastic container with a wide mouth which has about twice the capacity of the acid you will be putting on the glass. I use a very large photographic acid tray under the spout to catch the acid at the end of the etching. Always use new acid when French embossing. Acid which has been used once already leaves a slightly more obscure finish than clean acid. This comes from the residues of previous etching.

The prepared acid is poured slowly onto the glass, from a height of not more than three inches, to avoid splashes. We found that it required about four litres of acid to cover the 750 mm by 1525 mm sheet of glass to a depth of about 6 mm, which is all that is needed. These long etches are often called rotting. You will see at the conclusion of a three-hour etch (required at this acid concentration to etch to a depth of about half a millimetre) a build up of etching waste products into mould-like growths all over the etched portions of the glass. These do no harm, as the acid is working away underneath. I do not recommend any agitation of the acid to remove these, especially over areas of detail, as it is all to easy to lift the resist.

Emptying the Bath

When the etching is finished, put on all the respiratory equipment and rubber protective clothing, and gently agitate the acid with a small synthetic fibre paint brush to remove the mould-like growths. Make sure you have lots of spare water around, and some slaked lime or calcium carbonate close at hand to neutralise any spills. Remove all the miscellaneous tools that have collected near the glass and generally clear up the area. Then pull the plug and drain the acid into the waste acid container. This is very slow, so the wedges at the top are knocked in to lift the far end and assist the draining of the acid from the glass. When the last of the acid has dripped into the catching tray, put it into securely closed containers immediately! Avoid any possibility of tripping over it, or spilling it by taking the acid containers to the store cupboard and neutralise and rinse all the intermediate containers.

Cleaning and Finishing

The next step is to rinse the glass. Leave the frame and its glass elevated at one end and slowly pour several buckets of water over the glass. If you have access to a hose, you can irrigate the glass gently for about five minutes. All this waste water must be collected and neutralised before disposing of it. Calcium carbonate and slaked lime are good alkaline sources for this neutralising process. They can be added directly to the waste water, or a solution made to add as a liquid. When either is added, the waste water will effervesce. When this stops, add more alkaline and if there is no effervescence, the water is neutralised and can be disposed of in public sewage services.

To remove the glass from the frame, you need to break the wax seal, which can be done by running a thin-bladed knife along the edge of the glass. Then you reverse the process of getting the glass into the frame. When the frame is vertical you may need to break the seal of the wax at the rebate too. Someone must support the glass while you run a blade between the glass and the rebate on the bottom of the frame (top edge first). The glass, freed at the top side, will tip out of the frame very easily.

Now you can reveal all your work! Peal off the resist and have a look. The glass will need a clean up with alcohol or methylated spirit) and a soft brush to get rid of the adhesives. This must be followed up with water containing a very little soap and a brush to remove any remaining oils. Finally, frequent rinses with plain water are required.

If after this thorough clean up, you find some areas which are not ground enough, or worst of all, some acid has seeped through your best prepared resist, you can grind the surface again in localised areas. It is best when working close to an etched area, to cover the etched portion with resist. This is easy, because you can put the resist over a wider area and use the edge of the etched area as a guide for cutting the resist.

Biting and grinding is not a quick process, but it provides an excellent replacement for the unstable white acid, and is relatively safe. It can produce superbly detailed etched windows, allowing large amounts of light in, while maintaining privacy. It certainly has been a process of discovery for us!

July 1999