Tech Tips — Hot hide glue and making your own glue pot The advantages of hot hide glue in piano work: Reversibility: Hot hide glue is very water-soluble, simplifying future repairs Controllable penetration: Hot hide glue gels quickly, so it won't penetrate too far into porous materials like felt and harden it excessively. Actually, hot hide glue has a two stage curing process. First, it turns from a liquid to a gelatin as soon as it cools slightly. Then over a longer period of time, it turns hard and rigid as the water content leaves the glue. Sizing properties: Hot hide glue contains water, and this moisture causes bushing cloth to conform to shape when pressed into key mortises, flange bushings, etc. It also swells knurled hammer shanks for a tight fit in hammer bore holes. When the glue later turns hard, the parts are locked into the desired shape. Thus, precision key bushing cauls and hot hide glue yield bushings that require little or no easing. Adjustable gelling time: The time between application of the liquid glue and when it gels can be adjusted by the addition of urea, a dry granular substance available from fertilizer stores or from Pianotek Supply Co. (800-347-3854). This allows you to tailor the working properties to the specific job and environment. For instance, if the work area is cold, the glue may begin to gel before you've assembled the parts, bushing cloth, etc. And when hanging hammers, you may want to add urea to allow more time for alignment before the hammers lock onto the shank. High strength: Hot hide glue makes wood joints stronger than the wood itself, and because it dries rigid it will not creep under load. Indefinite shelf life of dried glue: As long as they are kept dry, dried glue crystals will last virtually forever, unlike liquid glues which deteriorate badly with age. Simple to mix and use: With the right glue pot, hot hide glue is a pleasure to use. With the wrong setup, it can be a nuisance. Make the world's best hot hide glue pot for under $20.00 Many technicians have tried hot hide glue with poor results. I believe this is entirely due to using the wrong glue pot setup. The Rival Hot Pot Express (box pictured here) has several advantages over the expensive commercial models, and when used as described here it will make hot hide glue a joy to work with. You can buy this item at many discount stores for under $20. The first secret of using hot hide glue is to mix the glue in a small container, then place this into the glue pot and surround it with a water bath. Choose a container proportional to the amount of glue needed so the glue mix is about 3/8" to 1/2" deep. To prepare glue for key rebushing, add glue crystals about 3/8" deep in a small jar or can, then add water into the glue container until it just covers the glue. Then add water to the pot outside the glue jar, equal in depth to the glue level. One advantage of this pot is its temperature control shown here. Hot hide glue should be kept at 145 to 150 degrees F. It needs to be hot enough to be well dissolved but not so hot that it breaks down. Overheating or holding at high temperature for a long time gradually deteriorates the glue. The first time you use your pot, place a cooking thermometer in the glue mix and adjust the control knob for 145 degrees. Then mark the pot so you know where to set the dial in the future. The most important requirement for a hot hide glue pot is that it have a lid in place at all times when dissolving and using the glue. This contains a steamy moist air over the glue surface, preventing it from skinning over and maintaining a constant consistency. As shown here, just use a hole saw to drill a 1" hole in the top of the pot. Each time you withdraw the stick or brush, you can wipe excess glue on the side of the hole and it drips right back into the glue container. Remember that hide glue gels as it cools, so without a lid the top surface of the glue cools and loses moisture, forming a skin unless constantly stirred. Do not even try to use any glue pot without using this water bath/ lid system or you will become discouraged. Stir the glue occasionally as it dissolves. It should be ready to use after about 10 minutes. Check the consistency. If too thin, add more glue crystals and wait for it to dissolve. If too thick, simply pour a little of the heated water into the glue container. For key rebushing, the glue should be about the consistency of heavy cream as shown at right. For hammer hanging, it should be slightly thicker to avoid dripping. As you begin to use the glue, pay careful attention to the gelling time. If it gels before you have a chance to assemble the parts, add urea ( a nitrogen fertilizer found at garden stores). Use 1/2 tsp. per Tbs. of dry glue crystals or more as needed. This is especially helpful in cold shop conditions, or whenever you have a slower assembly time. Additional tips: Glue strength gradually declines with extended heating or repeated cooling and reheating. You can successfully reheat hide glue several times for non-critical work like gluing on dampers, back rail cloth, etc. (store covered in the refrigerator between uses). However, for high strength uses like wood working or hammer hanging, always mix it fresh. Common lore is to mix the glue crystals with cold water only, then let it sit overnight before heating. I believe this is unnecessary and have always mixed and heated immediately with excellent results. Hide glue crystals and urea are available from Pianotek Supply Co. 800-347-3854
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