An improved method of ornamental glass chipping developed by the Rawson & Evans Company of Chicago in the 1890’s. This method is time saving and allows multiple techniques without the need of remasking or recutting as well as intricate selective chipping with a bare minimum of overchipping.

Although this tech sheet is basic, it will guide you through the steps for optimum results. The only materials change from the original patents is the use of modern sandblast resist instead of lead foil. Remember that this is a natural process and to quote an old saying, “A dog only knows how to be a good unless he’s sick”. We have used & taught this method for many years and I’m sure you will find it both a rewarding & profitable endeavor.

STEP ONE (mixing the glue)
NOTE: Only one type of glue will work for this process. It is a low grade, dry, animal hide glue with an approximate 165 gram strength and designated as glass chipping glue. High strength hide cabinet glues or gilder rabbit skin glues are very elastic and will not work. Neither do white or yellow wood working glues.

Using a 3 oz. Dixie cup as a gauge, take 1 cup of dry glue with 1½ cups of cool tap water for every approximate square foot of area to be chipped. Mix these together in your glue pot and cover with a plate or scrap of glass to prevent water evaporation. Allow to soak at least 30 minutes before any heat is applied. A double boiler works fine, but you will soon tire of the thermometer mess and upgrade to an electric glue pot. It’s important that the glue temperature does not go above 160 degrees or else the glue becomes cooked, rendering it useless. An electric pot has a built in rheostat to maintain the correct heat. The water to glue ratio of 1 1/2 to 1 is sufficient for most selective chipping. Remember that the more water you add, the more that has to evaporate before it can chip. Using less water in the mix (1 to 1) slows the glue flow and gels rapidly. This can cause difficulty in achieving an even glue layer, yet is invaluable when brush applying multiple thin coats to curved or cylindrical objects such as neon tubes or jars. On the other hand, when large solid areas are to be chipped (say 4 sq ft or larger) you want the gelling slowed down to level out evenly. Here we will use as much as a 2 to 1 ratio. It can take several hours to gel, but as long as you have the glass panel perfectly level, the glue thickness will be consistent.

STEP TWO (preparing the glass)
Virtually any type of glass can be chipped in any thickness with the popular choice being ¼” thick annealed plate. Double strength (1/8”) is fine for smaller projects but single strength window glass (1/16”) should be avoided in any size over a square foot. If you attempt to chip large areas on thin glass, you risk cracking the panel during the glue’s contracting phase. ¼” or thicker will eliminate any chances of disaster. Clean the working side of your panel with Bon-Ami or soap and water to remove all dirt and cutting oils. Windex type sprays are fine for this step as only general cleanliness is required. The use of a short wave ultraviolet light to determine the good (uncontaminated) side of the glass is only recommended if you plan on chemically applying gold or silver to the piece.

With your panel cleaned and dry, apply asphaltum varnish in an even thin layer to your working side. If only border work or center work is desired you need only cover those areas. A short nap paint roller is ideal and when finished, submerge in a can of water. As long as the roller remains under water, the asphaltum will never dry. (Before the next use, just drain and roll off ALL moisture and you’re ready to go). After 10 minutes or so or when the varnish film feels dry to the touch, apply your sandblast resist or vinyl to the surface. DO NOT use application fluid. Apply dry avoiding air bubbles as best as possible. Intermediate 4 mil vinyl is sufficient as you are only going to frost the glass. Our favorite is 3M Scotchcal Plus #3470. It is very elastic, economical, 4 to 5 mil in thickness and is great for both light carving and frosting. It plotter cuts well but only comes unpunched. After squeegeing well, pounce on your design and lightly spray with clear lacquer to fix it. Knife cut the image and weed out the resist. Those with plotter capability can just transfer your premasked vinyl as normal.

STEP THREE (sandblasting)
All that is required at this step is a light frost, just enough to remove the shine from the surface. The asphaltum layer will disintegrate on contact with the abrasive spray. A cheap syphon sandblaster will work but this is slow and not as preferable as a pressurized system. If you don’t blast in-house, a local art glass studio will usually accommodate you. Avoid large sandblasting services which descale metal or blast wood because their pressure is too high and the abrasive size too coarse. Silica sand in the 100 to 120 grit range is adequate with 120 or finer grade preferred in the more expensive aluminum oxide or silicon carbide. After blasting, blow off any loose dust on the surface. Do not wipe it off and avoid touching the abraded surface lest you contaminate it with finger oils.

STEP FOUR (leveling & glue application)
If you HAVEN’T already, plug in your glue pot or turn a low fire on your double boiler. I might mention that you can use a microwave oven to heat the glue as long as you give it short bursts and don’t bubble it out of the container. Whichever method is used, at the point where it is soupy with no remaining lumps when it is stirred, it’s ready to go. Technically, the glue mix is optimum when it has been taken between 140 and 145 degrees for 5 minutes. This lines up the protein chain and delivers it full strength and is referred to as prepared gelatin. Any time span beyond this preparation only requires heating to a flow able state.

Place the glass panel on a comfortable working bench and strip off the sandblast resist, taking care not to disturb the varnish film at the perimeter of each frosted area. If the asphaltum readily peels off the glass along with the mask, then either the glass was not clean or water moisture was involved during its application. Just touch up any voids with a brush. Now, using a bubble level and some wood wedges or ice cream sticks, level the piece as accurately as possible. This will insure an even glue thickness and obviously the larger the open areas to be chipped, the more critical the leveling. Going to the glue pot, stir one final time and fill up the squeeze bottle. You can wrap a shop or paper towel around it as it is usually hot to handle and place the cover back on the pot. Squeeze the glue into the open areas as lava would flow. The harder you squeeze, the more material flows onto the surface. With a little practice you can easily trace the flow to the outer edges as well as filling in the center voids. Notice that the glue will self level to a perfect desired thickness or approximately 1/16”. The asphaltum acts as a chemical dam rather than a physical one, as water and oil DOESN’T mix. This repels the glue at the edges and eliminates moisture from creeping under a mask, causing a ragged chip. For the time being, don’t worry about any glue flowing beyond the edges as this can be easily trimmed off once it gels. When finished, return any leftover glue to the pot and thoroughly wash out the bottle and while still filled with water, replace the nozzle and flush it. Any glue remaining in your pot can be transferred to a sealed plastic container and refrigerated until its next use. Even months later, it need only be warmed to a flowing state for use.

STEP FIVE (drying and cleaning)
If you wish, place a fan blowing gently over your piece to aid in its drying. When the glue has reached a cello-like state, trim off any excess that has flowed beyond the image with a wooden wedge or razor blade. Any asphaltum trimmed off at this point is unimportant now. Allow the glue to dry to a tough hard stage. If you live in a dry climate, the glue will not only dry rapidly on its own but go through the entire chipping stage without further help. For the majority of us who have humidity levels over 50%, alternate action is required. Once thoroughly dry, the piece can be exposed to the hot sun, warming up the surface and driving off the moisture. This works relatively well providing the sun stays bright without cloud cover. We have found without fail, rain or shine, that the use of a drying heat box is the best. You will find a diagram of its construction at the end of these notes. The optimum humidity is 35% or below. Between 35% and 50%, chipping will occur although slowly. If the glue layer is too thin, it won’t be strong enough to chip. This is usually evident by the glue surface having a rough texture, as coated sandpaper. Remedy this by wiping over the surface with a water dampened paper towel and (by brush or nozzle) reapplying a fresh thin coat of glue. If some areas of glue are too thick, then they will take much longer to chip. If a thick area still refuses to budge, soak it in water to swell and remove the glue. Then, taking care not to contaminate the frosted surface, paint an outline of the asphaltum around the unchipped area and reapply some fresh glue. . The reason for this outline is to contain the glue and prevent a double chipped area. During the normal chipping process, there occurs a desire on our part to help the glue along by picking at the pieces to knock them loose. This can cause over chipping at the edges and should be avoided. Tapping at an unchipped area with an X-acto knife can satisfy this need as well as giving the glue a fresh fracture to work with. When the process is completed, remove the panel to a flat surface and soak in water (wet newspapers will do) to soften and swell any remaining glue, allowing it to be washed away. The remaining asphaltum can now be easily and quickly removed with paint thinner or naphtha. Wash off the residue with soap and water and dry.

ADDITIONAL NOTES
When chipping large solid areas or complete sheets of glass, we have grooved plastic rollers available which act as spreaders, leaving the correct thickness of glue on the surface.

Double chipping is accomplished by a second application of glue after the first one is completed. This second layer attaches itself to the sandblasted ferns remaining, tearing these out and resulting in a nugget appearance.

Oceania has a wavy look and of having large sections ripped out and is usually reserved for divider panels. When your glue film has gone beyond the gelled stage and yet retains a damp cement feel, carefully place a sheet of heavy bond or butcher paper onto the surface. If pieces of paper overlap one another, tear the underlying edge ragged to avoid straight lines from appearing in the finished work. The drying time can be a little longer, but as a result, the layer of stuck paper will curl up from side to side giving an ocean appearance.

At times you may have simple bold designs and choose not to use the asphaltum. In these cases, after you have masked, weeded and blasted the piece, level as usual and apply your glue. Once it has reached the gel state, trace around the design with an X-acto knife and strip off the mask.

(Thanks to John Jordan for supplying this document to this site, and also to Kimberly Zanetti for retyping the old document for use here!)