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Etch Masters: Understanding Resist In The Glass Business

Glass etching is the business of creating images permanently engraved on glass. In this business, resist is one of the most important elements. It is one of the main keys to the difference between success and failure.

Design is the image you engrave on the glass, and is clearly an important part of the process. Etching technique actually produces the image on the glass, but the understanding, selection and control of the resist controls how the designed image can actually be produced.

Resist is the material that protects the surface of the glass where you don’t want anything to be etched, when you blast the surface with abrasive. Though there are many different kinds of resist, they all share several characteristics—they are thin, flat sheets of resilient material (like vinyl or rubber) that stick to the glass and block the impact of abrasive on the glass. The design is created by revealing portions of the glass surface to the effects of the blast while protecting all other portions with the resist. The parts of the surface that are revealed are the areas of the design that you want to be etched.

You reveal the portions of glass to be etched by cutting and removing the resist from those areas after applying the resist to the entire surface of the glass, or by removing those areas of resist before applying it to the glass. The resist you choose, and the way you cut and remove the areas of resist depend on the type of design you will be etching, and a thorough knowledge of etching techniques is very helpful to deciding what type of resist to actually use.

Resist can be removed from the glass all at one time, and the glass is blasted only once. This is called single-stage etching. Single-stage etching can be blasted only on the surface (and the etching technique is called surface etching) or it can be blasted deep into the glass, when it is called carving. Elements of the resist can be removed in a sequence (instead of all at once); with portions of the design being blasted after each time resist is removed. Sequential removal of resist and subsequent blasting is called multi-stage blasting. Stage blasting can be deep in the glass, and the technique is called multi-stage carving, or it can be so light on the surface of the glass that the glass is barely etched. This is called multi-stage shading.

Single- or multi-stage resist removal, along with the depth of blasting, affects how each element in the design is visually separated from the other elements that touch it, which determines how the design looks after blasting. In other words, a surface-etched design looks completely different than a multi-stage carved design, and they both look different than a multi-stage shaded design.

The style of etching used for a particular project not only determines how it looks, but how much time it takes to finish the project. Multi-stage etching takes longer than single stage, and carving takes longer than surface etching or shading. So, if you know how to produce all the different effects of these different blasting techniques, it not only gives you the ability to create etchings with vastly different appearances, you can also arrange your offerings to your clients so you have a wide range of prices.

There are two major categories of etched products being sold today: The first is large architectural pieces such as window, door panels and table tops, and the second is smaller pieces such as gift items, personalized products and awards. The category indicates to some degree what types of resist you might want to use.

VINYL OR RUBBER ROLL RESIST

This is the type of resist you will probably use if you are doing architectural etching or signage. The choices of this resist will depend on whether you are blasting shallow or deep and how fast you like to work. In general, rubber resist is tougher than vinyl, and thicker layers of each material are tougher than thinner layers. Vinyl resist can be divided into hard vinyl resist (like sign vinyl, but thicker) or soft vinyl (acts more like rubber resist). Since soft vinyl acts more like the rubber resist, we will consider it in the rubber-resist category. These resists come on rolls of 15 to 36 inches wide and 10 to 50 yards long.

Vinyl blasting resist is available from 4 mils (a mil is a thousandth of an inch) up to 15 mils thick, and rubber resist is available from 11 mils to 45 mils thick. Vinyl is most commonly used for surface etching and shading, and for moderately deep carving (up to about 1/16” deep in glass). Rubber resist can be used for surface etching (though it is more expensive than vinyl), but is most commonly used in carvings over 1/16” deep.

In addition to the depth of the cut into the glass, you have to consider how fast you want to work, when picking resist. For example, if you want to really take your time on a finely detailed design, so you can control the carving, you might want to etch a given design with the nozzle 4” away from the surface, set the blaster at 30psi and take your time. On another design, where there isn’t a lot of fine detail, you might want to hold the nozzle at the same distance, but turn the pressure up to 60psi. Then, you will be cutting into the glass at least twice as fast, and the etching will take only half as long. In the first case, you could easily use an 11 mil soft vinyl resist, but in the second case, you would have to use at least a 25 mil rubber resist.

If you are working on stone, metal or wood, you will also have to select a thicker, tougher resist because in general, your pressure will be higher than on glass.

Once you have decided on your resist, you have to produce the image to be etched. This type of resist is not photo sensitive, so it can’t be photo processed like photo resist. The resist itself must be either hand cut with a stencil knife or laser cut or plotter cut. The laser might not be an option because most of these materials contain PVC, a no-no for lasers.

Hand cutting is by far the most common way of producing the image to be blasted, since most of the architectural etchings are designed specifically for one job and might not be repeated again. Yes, it does take a little time to learn to hand-cut resist accurately, but you might be surprised just how fast you do learn.

If you will be doing the same design many times, it might be worthwhile to machine cut the resist, if you have access to a cutter that will cut heavy materials. Keep in mind, though, that just scanning in the design, cleaning it up and vectorizing will take more time for one etching than tracing and hand cutting will.

Since it is very difficult to hand-cut lettering or fine detail in geometric designs accurately, you may want to combine the hand-cut design with a photo-resist insert to accommodate this situation (we’ll show you how to do this in a future article).

PHOTO RESIST

This is the type of resist you would choose if you were etching awards, corporate gifts, personalized glass items, or a production gift line. Photo resist performs the same function as hand-cut roll resist, but the thicknesses of the two types of resist vary considerably, and the processing of the image onto the photo resist is totally different. Photo resist tends to be tougher and more blast resistant than the roll resists per mil of thickness. So, a 5 or 6 mil thick photo resist can be as tough as a 10 to 12 mil thick rubber resist.

The major reason you would select photo resist over roll resist would be in the ease of producing an extremely accurate resist image, with very fine detail possible, without the need for any cutting of resist at all. In addition, it is also the choice when you are producing a large number of small-to-medium-sized items, all with the exact same design on them. The process of cutting is completely replaced by exposing the photo resist to ultraviolet light, with the original black and white image on it, then washing out the image. The blast-resistant emulsion is water soluble until exposed to UV light, so any areas under the black portion of the design do not get exposed, and therefore, they can easily be washed away after exposure.

At least that is the procedure with the type of photo resist that is designed to be processed with water. Another type is available that is considered to be a dry-processed resist, which does not require water. This resist is also processed with UV light, but the original artwork it requires is the opposite of that required by the water-washout resist. In other words, if the water-processed resist requires a positive printout from your computer, the dry resist requires a negative. In this case, it is not the black areas that get etched but the white areas of the original.

There are a number of requirements to fulfill to get a good photo-resist image, like getting good original artwork printed out from your computer, the correct printout substrate to use, the right exposure time, the right washout time (for the water-processed resist), etc. However, the same basic principles apply with regard to selecting resist based on thickness—the deeper you want to etch or the faster you want to do it, the thicker the resist needs to be. Photo resists are available from 2 mils thick up to about 10 mils thick. However, you won’t generally select a photo resist solely based on thickness.

The selection process depends on several other factors. First, do you have warm water available for processing, and do you want to spend the extra time that water processing requires in order to get its benefits? Second, do you prefer working with a resist that is self-adhesive or one that requires a separate adhesive? Third, do you plan on blasting in a single stage or in multiple stages, removing resist between stages?

The benefits to the water-processed resist are that you can make up a large number of resists in advance, and you can store them for months, using only a few at a time. Once they are finished, they can be applied and blasted in normal room light or bright sunlight, and they can be exposed to light without regard to ruining the material. They are also available much thicker than the dry-processed resist, so you can use them when you need to blast a lot deeper.

When using the resist that is self-adhesive or when using a dry-transfer adhesive, these resists are much easier to blast in multiple stages. Non-adhesive resists are more trouble to use, since you have to apply adhesive separately, but they can be good choices whenever you have a slightly rough substrate like ceramic tile or unpolished stone (such as river rocks and flagstone), or when adding detail to an already etched glass surface.

It is difficult to make up a large batch of dry-processed resist in advance, for future use, because it is light sensitive throughout the blasting process and degrades within a couple of days after exposure. It also can’t be left out in regular room light for more than an hour or so without degrading. For both these reasons, when doing a job with this resist, you should plan to only expose enough resists at a time that you can complete applying them and etching the glass within a one-to-two-day period.

Even then, the resists need to be kept in a restricted-light situation until the glass is blasted. This resist is self-adhesive, but difficult-to-impossible to peel and blast in stages because of the tenacious adhesive. Last, when using this resist, it takes quite a bit longer to etch the glass, because you have to blast through the exposed resist before the glass becomes etched.

So when would you use this resist? It does have some significant advantages. First, it is fast and easy to use to produce an etching that requires quick turnaround. If while-you-wait service is one of your specialties, this is the resist for you. It is also great for limited production runs, when you can easily expose the resist and etch the glass all at the same time. The last major benefit it has is its ability to reproduce halftones on the glass. The 2 mil version of this resist is capable of reproducing halftones of 35 to 45 line screen resolution. With great care, a lot of practice, and very fine abrasive (220 or finer), you may be able to get even finer halftones, say 55 or 60 lines per inch.

We find that many people who get into etching or carving on glass start out with just one or two resists and then stick with those resists for every job. As you can see from this discussion, there are many considerations you have to take into account to get the best resist for the job, whether your job is on large glass or smaller items. If you use the wrong resist, it will take you longer to get good results, you may waste a lot of resist and possibly a lot of glass, and you may be wasting your most-precious resource—your time! We highly recommend that you try a good variety of resists that are available, with an open mind, to learn of their benefits for your process.

© 2007 Norm & Ruth Dobbins