Saturday, 17 September 2011

Leading Basics

Leading up boards
It is often best to have a separate board to place on top of your bench to do the leading. This means that you can move the project if it has to be delayed while having to do something else.
Start with a work board that is thick enough to be relatively rigid, but is easy to put nails into. Plywood is a good, but relatively expensive board. MDF is heavy and difficult to put nails into, so avoid it.
You can either have two permanent battens of wood about 19 mm thick attached at right angles to each other in one corner, or you can attach them to the board as required for each project. The permanent placement means you do not have to check the accuracy of the right angle each time you use it, but it does not allow easy adjustment for smaller or larger pieces than the battens will accommodate. The temporary solution requires checking the right angle each time you use it, but it allows you to place the battens over the cartoon at the appropriate distance from the battens without cutting the cartoon or making multiple checks of the accurate placement of the cartoon.
The battens should be attached to "base" board about 60 mm in from the edge to have a little work area to cut leads, etc. They need to be a little longer than the dimensions of the pattern you are assembling. 

Corner battens nailed down over cartoon and perimeter leads placed

If you are putting the battens on top of the cartoon, you can use the cut lines to align your battens. Cut two short pieces of the came you are using for the edge. Centre their hearts on the cut line and butt the battens against them. Nail or screw the batten in place. Repeat for the other side. 
Using piece of came to set the perimeter batten

If you are using permanently fixed battens, place the cartoon, which has been trimmed to the outside lead line on two sides, against the wood strips. Use some horseshoe nails or tape to hold the pattern in place. Check to ensure the correct distance has been maintained between the battens and the cut lines on the cartoon. Adjust as necessary.
Establishing the Perimeter for a Window
The first thing to be established about the panel is the placing of the came that goes around the edge of the panel.
Make a straight cut across the outside came and put that trimmed end into the corner and along the vertical wood strip. The lead should be longer than the leading cartoon to accommodate the length of the upper horizontal. If it is even longer than required, the extra can be trimmed off after soldering. 
Came butted at corner
Next butt a trimmed piece of perimeter came along the horizontal wood strip. This one should be shorter than the cartoon. It should be shorter by half the width of the perimeter cames to allow the vertical came to butt against it. An easy way to do this is to use a short piece of the appropriately sized came. Place the heart on the cartoon cut line and next to the perimeter came. Use you lead knife to extend the inside of the came across to mark the perimeter came. 
setting for cutting the bottom perimeter came to length

Placing a piece of came to set the line for cutting
The reason for having the vertical cames running from bottom to top is that there is a fraction more strength in the heart of the came going all the way to the bottom of the panel, rather than resting on the flanges of the came.
These perimeter cames should be held in place with horseshoe nails. Try placing the nails only where a lead line will be soldered in order to cover any nicks the nails might make. Alternatively, you can place the nails at the ends of the perimeter cames to keep them from sliding vertically or horizontally.
Straightening the Came
Before using the came it is important to straighten it. This increases the stability of the came during the leading process. Most often nowadays, you use a lead vice. This operates similarly to a cleat on a sailing boat. The more strain that is applied, the tighter the vice grips the came.
You place the end of the came into the vice so that the came appears at the back of the vice. Give the top of the vice a firm tap with your pliers to set the teeth into the came. Grasp the other end of the came with the pliers, and put one foot behind you to brace yourself if the came does slip out of the vice. Draw the came toward yourself until you can see the lead is straight and any kinks have straightened. 
Short piece of came being straightened with lead came vice and pliers
Take the came out of the vice and keep it straight. You transport it by grasping each end and keep the came under tension until you get it to the destination. It is often easiest to cut the full length in half before moving it, as it will not then be longer than your arms can stretch.
Remember, this process is to straighten the came to give pleasing lines in the leaded panel. It is not stretching the lead. Stretching the came can weaken the lead.

Dressing the Came
This relates to how easy it is to slide the glass into the channel of the came and nothing about your clothing.
If you have consistent difficulty in sliding the glass into the came, you should consider dressing the came before use. Dressing the came consists of running a fid or other hard material along each of the four flanges of the came. In doing this, you are pressing each flange down against the bench or other smooth surface. 
Pressing the came flange down with stopping knife.  Each of the four flanges need to be treated in this way

Dressing the cames gives a slight bevel or ramp for the glass to slide over the edge of the came and into the channel of the came. You can dress the whole length at once, or as you cut the pieces off from the main length. Dressing shorter pieces is less likely to bend the came away from the straight.
Of course, it is not enough just to dress the came at the start. There is an analogous procedure after the whole panel has been leaded, soldered and cemented.
In this instance the term ‘dressing the cames’ means to close or bend the leaves/flanges of the came toward the glass. This is done after the soldering and cementing is complete. It provides a neat rounded appearance to the lines, traps the cement you have already added, presents less area for the rainwater to collect, and makes polishing easier. It is also the time when you may break the glass by putting too much pressure on the glass, so be careful!
Dressing the cames at the finishing stage is done with an oyster knife or fid. It is often best to avoid metal and better to use wood sticks or plastic tools. The pressure is placed on the came rather than the glass. Run the fid lightly at a shallow angle along each flange of the came. It is helpful to use a finger of your other hand to guide the fid along the cames. You may want to do this several times, as repeated light pressure will cause the flanges of the came to move gently toward the glass with less risk of breaking the glass. This can only be done while the cement is pliable. If it is done after polishing, you will need to re-do the polishing, as it will make the edges of the came silvery rather than shiny black.
Cutting Lead Came
Cutting came is a gentle process rather than an abrupt chopping effort.
There are at least three kinds of implements in common use to cut lead came. 
Clockwise from the top: curved blade (pro), straight blade, lead dykes or nippers

Lead nippers or lead dykes
Lead nippers/dykes are a kind of adapted side cutters, used for cutting wire and by electricians. But leaded glass dykes have the bevel only on one side of the jaws, making them almost useless for anything other than cutting lead. This arrangement only crushes the lead on the cut-off side and also leaves a minimum of lead next to the back of the jaws.
To use them the jaws of the dykes are aligned in the same angle as the heart of the lead, cutting across the leaves of the lead. They do not cut from the top and bottom of the came. These are very quick for right angle or very oblique angles on the came. However they are of little use for acute angles.
Lead knives
For more acute angles, blades are more commonly used. These can be either straight edges or curved blades. The straight edge lead knives are essentially putty knives or stiff scrapers sharpened to an acute angle. This kind of knife is normally wiggled from side to side while applying pressure to work through the came.
Other knives are curved to make rocking back and forth easier. There are a variety of knives such as the Pro or Don Carlos. Some look more like a scimitar than a lead knife! These are used to rock along the line where you are cutting the came.
What ever kind of knife you are using, be sure to be directly above the knife, looking along the blade to ensure vertical cuts.
Of course, saws are sometimes used. The blade needs to be coarse toothed to enable the soft lead to drop out of the teeth. These saws can be hand held or table saws. Normally, it is quicker to use lead dykes or knives. However, if you are in production mode, a powered table saw may be worthwhile.

 Leading Procedure
Cut the leads exactly as the cartoon indicates. In other words, where one lines runs into another, that is generally a stopping/starting point for the came.
Always lead to the cartoon line, not the glass. This ensures accurate completion of the panel. If the glass is slightly too small, the cement will take up the gap (assuming the flange of the came covers the glass – if not, you need to cut another piece of glass that fits). If the glass overlaps the cut line, it needs to be reduced.

Placing gauge for cutting to length
Cut the ends of the came shorter than the glass. The best way to determine the extent of this is to place a piece of came of the dimensions being used for the next edge on the cut line. Use it to determine the length and angle for the cut. The object is to have each piece of came butt squarely against the passing came, to make a strong panel and to make soldering easier. 

Marking the came for cutting to length

Testing the length of the came

Leading the first pieces
After establishing the perimeter cames, place the first glass piece into the corner formed by the perimeter cames. Normally, you will be working from the lower left corner toward the upper right corner of the pattern if you are right-handed. The reverse is the case for left-handed people. Hold each piece of glass in place with some scrap lead and nail. The scrap lead will prevent the nail from chipping the glass. It's important all glass is held in place with nails so no shifting occurs while working in another area of the panel.

Fitting the Glass to the Cartoon
Often you find that the next piece of glass does not fit properly. Possibly it rocks a bit in the came’s channel. Possibly it is simply just a little too big.
Glass too wide and slightly angled along the long line
The first thing to do is to take the previously placed piece of glass out and remove the came, to ensure the previous piece of glass is not too large. The glass should not overlap the cut line. If you have drawn your cut lines to 1.2 mm you should see only the faintest line of paper between the glass and the dark cut line. 
If the glass seems too large, check that it is firmly in the channel of the previous came, as sometimes the glass catches on the edge of the came and does not go into the channel.
The next check is to determine whether the apparently too large piece of glass really fits the cartoon cut lines. Place the glass inside the cut lines. You should see a faint line of paper between the glass and the cut line.
When you are sure both pieces of glass are the correct size, put the came back between them and check again. If the glass is still too large, make sure the came butting onto the came separating the glass is not too long. This is a common reason for lead panels to grow beyond their initial dimensions.
If the glass is the correct size and the butting cames are correct, replace the came. Put the too large piece of glass into the came and position it so it has the best fit to the next cut line. Take a felt tipped pen and run it along the edge of the came, marking the large piece of glass. 
Marked glass with nail pointing to area too wide that needs reducing
Take it out and check on where the line is farthest from the edge of the glass. That is where you need to reduce the piece. 
Snugging the came to the glass
It is important to have the came fit snugly to the glass (assuming the glass to be the right size and shape). If it does not, the panel is likely to grow beyond the intended dimensions.

To ensure the came is tucked snugly against the glass, you use a fid of firm material (wood or plastic, for example) to press against the heart of the lead. You can press directly toward the glass, or make multiple passes along the length of the came to ensure the heart is touching the glass all along its length.
You should avoid steel tools, because you may cut the lead, and if the blade is wide you will not find it easy to fit along all of the curves.

Following all these steps in the process of leading should lead to quickly and accurately assembled panels.

Wednesday, 31 August 2011

Drilling Glass

Keeping Things Wet
When Drilling glass it is important always to keep the drill bit and glass wet, otherwise the glass gets too hot and will break. There are a variety of things you can do to achieve this.

Drill under water in a container
Drill in a ring of clay, plasticine, etc., holding water. To do this, you need to make a ring about 50 mm  in diameter and press it around the drill site. Fill the ring with water and a little diamond coolant. This will cool the drill site and glass. Diamond coolant is not necessary, but extends the life of the bits.
Use a re-circulating water pump such as those made for indoor water features. Direct the small flow of water to the drilling site and catch the water in the bucket in which the pump is submerged.
Use a glass drill with hollow core bits and an internal water feed. This is the most expensive but it is the best equipment with which to drill large holes.

A Typical Drill Press Set Up
Drill Press
It is best to have a drill press if you are doing a lot of drilling. It provides a stable drilling action and the pressure on the bit can be controlled. It is important to ensure the bit is running true without wobble. The drill press should have instructions to help correct any untrue running of the chuck. Make sure the drill bit is secured firmly. Core drill bits are easier to keep true, as they normally have a threaded fitting.
With a drill press, you can drill continually until the hole is completed, or until a white paste or dust begins to appear. This indicates the drilling is being done dry and will in a few moments heat up the glass too much. When the white paste appears, back out of the bottom of the hole a little to allow water to flush the glass out. Then continue.

Keep a firm grip on the glass being drilled. Maintain its position, especially if you are intending to back off intermittently to allow water to the bottom of the hole. This enables you to get back into the hole without scratches.

If possible, submerge the piece if you are drilling without a core drill bit. But if that isn’t possible, just squeeze a little puddle of water on the surface and watch it swirl around. You can see if it is pulling ground glass out of the hole by watching the circulation. Placing a plasticine or clay dam around the drill area will keep the water confined.

Don't push down any harder than you comfortably can with the tips of your fingers. Keep it steady. Listen for the sound of diamond grinding glass

White core stuck in the drill bit

If the core gets stuck in the bit, knock it out with some stiff wire or a nail. Always remove each core right after drilling. They are very difficult to remove if there is more than one in there.

Core pushed out with 16 gauge copper wire
Every diameter drill bit has an optimum drill speed. The smaller they are the faster the speed required. Based on what the manufacturers recommend, a Dremel running at top speed is way too fast. When using a Dremel for drilling glass, slow it down with the speed control. Drill presses do tend to be on the slow side for glass drilling so it takes a bit longer, but there are big advantages in other respects.

Pump (black) at the bottom and flushing head where the water enters (chrome) at the top
Drilling with a Flushing Head
A flushing head with a re-circulating pump will deliver water to the drill site through the core of the drill. These are supplied complete or as a fitting for an existing drill press. This is suitable for holes of 4 mm and larger. Smaller core drills are impractical both because the glass is easily trapped in the drill and the wall thickness of the drill makes them almost solid anyway.

An additional requirement is to have a means to direct the water back to the reservoir.

Every diameter drill bit has an optimum drill speed. The smaller they are the faster the speed. Drill presses do tend to be on the slow side for glass drilling, but often have ways of altering the speed. So it takes a bit longer, but there are big advantages in other respects.

Don't push down any harder than you comfortably can with the tips of your fingers. Keep it steady. Listen for the sound of diamond grinding glass

If the core gets stuck in the bit, knock it out with some stiff wire or a nail. Be careful not to damage the edges of the drill bit. Always remove each core right after drilling. They are difficult to remove if there is more than one in the core of the bit.
Avoiding Chipping
There are a number of methods to avoid chipping out the back of the glass when drilling:
Placing a piece of scrap glass under your good glass will help avoid chipping on the backside. By pressing firmly but gently on the glass (not the bit) the bit will go through the upper piece of glass without chipping the back. This can be a difficult process to keep stable when both the pieces of glass are wet.

Another method is to put duct tape under the glass to help minimise chip out.
Drill from both sides to avoid chip out as the bit breaks through. Go slowly toward the bottom of the hole. When the hole is almost through, turn the glass over and drill back to front.

Sometimes the glass is curved and drilling from the back is not easy. This is when the drill press mechanism to stop the bit comes into its own. Before switching the drill on, lower it to the surface supporting the glass. You can adjust the mechanism to stop the press just as it reaches the support surface. Then place the glass under the press and the turn the drill on to begin the drilling.
Don't push hard as you come to the end. Don't push down any harder on the drill press levers than you comfortably can with the tips of your fingers throughout the process. Keep it steady. Listen for the sound of diamond grinding glass.

All these things will help to avoid chipping out the glass at the bottom of the hole.
Drilling holes with copper tube and grit
You can drill holes by using loose grit and a copper tube of the correct diameter. It can take quite a while. You will need to have a chuck big enough to take the tube, or have a means to reduce the tube diameter to the chuck size. Alternatively, use core drills that have had the diamonds worn away.

Prepare the glass as for a drill press without a flushing head, so the water and grit are confined. The dam can be putty, plasticine, clay, or other mouldable material put around the area to be drilled.
The grit can be sandblast grit or other abrasive of about 100 to 200 grit.

Drill as normal.

If the core gets stuck in the bit, knock it out with some stiff wire or a nail. Always remove each core right after drilling. They are very difficult to remove if there is more than one in the core of the bit.
There are a variety of tools that can be used to power glass drills.
Dremmel and similar craft motors
These are light duty high-speed drills. Those with variable speed controls are especially useful. They work best for small diameter holes. They must have the speed turned down for drilling, especially for larger holes.

These can be combined with a flexible drive shaft for lighter weight.
Drill press
However, the most important thing to have when drilling glass is a drill press. Doing it by hand is very difficult and wears out diamond bits very fast. Dremmel and others make drill presses for their tools.

Drilling machines
Purpose-made glass drilling machines are important for larger holes and production work. Flushing head adapters are available from suppliers that will convert a standard drill press into a glass-drilling machine. These tend to be much slower than the Dremmel style motors, but are very steady. The important thing about these is that they use hollow core drill bits, allowing the water to be fed through the drill bit directly to the glass-drilling site.
Drill bits
The other tool needed is drill bits. The recommended type depends on the size of hole to be drilled.
Small diameter holes, up to and including 3 mm require solid bits. These can be spade (unusual) or solid diamond-tipped bits. A number of manufacturers make solid drill bits from 2-6 mm and some (especially lapidary suppliers) make the very small diameter bits from less than 1 mm to 2 mm.
Larger diameter holes are best drilled with hollow core bits, as less glass needs to be removed to achieve the hole. These can be used with a flushing head or simply by directing water to the drill bit, with a dam to hold the water around the site.
The bits will last longer if you use a drill press. The press keeps the bit wobble to a minimum and maintains the vertical, both helping to reduce the wear on the bit.

A selection of hollow core drill bits, wire and punches to clear the drill of stuck cores, and dressing stones
Hollow core bits
Hollow core diamond bits are of two types:

One -where a heating process attaches the diamond - is called sintered in Europe and a number of other countries.

The second – where the diamond is bound to the metal with resins and other chemical attractions – is called bonded in Europe.

Bits of the first type are longer lasting and more expensive. These can be dressed with an aluminium oxide dressing stick to maintain their effectiveness.

Bits of the second type wear quickly and should not be dressed.

In general a diamond core drill breaks out much less glass at the bottom of the hole than a solid drill bit.
Water pump
A further tool that is useful to have is a re-circulating pump. This can be a small fountain pump with a flexible spout to aim the water on the drilling site. A foot switch can control the water flow. A large tub is required to act as the catch basin for the water that comes off the drill and as the reservoir for the pump.
Drilling glass without a drill press
It is best to have a drill press for drilling holes in glass, but there are ways of doing it with a hand drill.
Make a ring of modelling clay, plasticine, putty or other mouldable material about 5 cm in diameter and press it around the drill site. Fill the ring with water and a little diamond coolant if you have it. The liquid will cool the drill site and surrounding glass as well lubricate the drill bit.

Adding diamond coolant to your water extends the life of the bits. Buying better (more expensive) bits is worthwhile as they work much better than the cheaper ones.

Use a paint pen to mark the spot where the hole is to be. Without a drill press, starting at an angle with a slow drill speed will stop the bit from sliding around as you establish the drilling point. As the glass surface is roughened, bring the drill to vertical. Move the drill up and down a little as you drill to allow the water into the hole. If you are using a solid or spade drill, a little oscillation keeps the bit from jamming in the hole. Do not do this with a core drill.

A Dremel running at top speed is way too fast. Slow it down with the speed control.
Every diameter drill bit has an optimum drill speed. The smaller the bit is, the faster the speed required.
Hole Placement
The general rule on drilling holes in glass is that the edge of the hole should be further away from the edge than the thickness of the glass. This means that the edge of the hole on a 6 mm thick piece of glass must be more than 6 mm from the edge of the glass.

The calculations are simple arithmetic. You calculate the centre point of the hole by adding the radius of the hole to the thickness of the glass plus at least 1 mm. For example, to drill a 10 mm hole in 6 mm glass, you add 5 mm (radius of hole) to 6 mm (thickness of the glass) plus 1 mm = 12 mm as the minimum distance from the edge of the glass to the centre of the hole.

Remember this is the minimum distance. For safety and durability in architectural or heavy circumstances, an additional margin must be added.
For methods of centring the drill see here

Drilling speeds for diamond bits in glass
Diameter -- Speed
3-4 mm -- 6000 rpm
5-8 mm -- 4500 rpm
9-12 mm -- 3000 rpm
13-16 mm -- 2500 rpm
17-25 mm -- 2000 rpm
26-28 mm -- 1800 rpm
29-44 mm 1500 rpm
45-64 mm -- 1200 rpm
65-89 mm -- 900 rpm
90-120 mm -- 800 rpm
[Based on CR Lawrence and Amazing Glazing recommendations]

As you can see the larger the diameter, the slower the speed. This is because you are attempting to keep the speed of the diamonds moving against the glass at approximately the same speed, regardless of the diameter. If you did not slow the speed as the diameter went up, the speed of the diamonds would increase, leading to overheating of the bit and reduction in its life.

Wednesday, 6 July 2011

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.

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
The Bath
Sealing the Bath
Emptying the Bath
Cleaning and Finishing


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!


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.


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.


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.

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