Over the past few years I have purchased several bottles of liquid hide glue. There are so many situations in woodworking that benefit, or even require, the reversibility afforded by hide glue. Plus, the long open time of liquid hide glue - almost 30 minutes in some scenarios - is a huge advantage. Plus, as I just mentioned, if something terrible goes wrong and things don’t come together in time, the glue is reversible.
I have stuck with Patrick Edwards’ Old Brown Glue over the years since that was the first liquid hide glue I tried and it has always worked well. My only complaint with it is the shelf life. Well … I guess it would also be nice if it smelled more like warm cinnamon rolls rather than wet dog. I would like to say that I get enough woodworking done in 6 months to use an entire 20 oz. bottle, but unfortunately that is not the case. The shelf life is especially problematic when I order a bottle of Old Brown Glue with items from other vendors (to save shipping costs) and I receive a bottle that is already several months old.
Recently, I began using hot hide glue as well. The quick, no clamp rub joint capability of hot hide glue is a huge advantage in some scenarios. I would also like to try my hand at hammer veneering at some point. Theoretically a person could use hot hide glue almost exclusively. However, during the winter in my semi-heated garage shop the shortened open time severely limits the number of scenarios in which it can be used. This is where liquid hide glue comes into play. Liquid hide glue is pretty much unfazed by the 50 degree winter temperatures in my shop.
My last bottle of liquid hide glue recently expired. I contemplated buying another bottle of liquid hide glue from Patrick Edwards, but at $20 plus shipping costs it is not cheap. So I thought to myself – Why don’t I just make my own liquid hide glue? Making my own liquid hide glue would give me the freshest possible glue and I can make it in any quantity I want to avoid spoilage. Almost all the liquid hide glue recipes I have seen call for only two ingredients. Patrick Edwards uses regular 192 gram strength hide glue and urea. However, many other people recommend using plain old table salt in place of urea. One proponent of table salt is Don Williams (see Woodworking Magazine Winter 2009). Since Don has an extensive background in furniture restoration, period finishes, and chemistry I trust that he knows what he is talking about.
The biggest variable in the making of liquid hide glue is the ratio of hide glue to urea or salt. To find the right formula I scoured the Internet as well as resources such as Stephen Shepherd’s book “Hide Glue – Historical and Practical applications”. I found several formulas for both Urea based and Salt based liquid hide glue.
Most people recommend adding urea in ratios between 5%-30% by weight relative to dry hide glue. The most common recommendation is 15% by weight. If you don’t have a suitable scale a common formula is 5 tablespoons of urea to 1 cup of dry hide glue. I don’t know how this compares to the weight formulas above since I don’t have any urea on hand at the moment to weigh. I weighed 1 cup of dry hide glue pearls at approximately 5.5 ounces. As a point of reference – there are 16 tablespoons in a cup. So unless Urea weighs significantly less than dry hide glue I would assume this formula is towards the high end (30%) of urea concentration. This is confusing considering the fact that one of the sources that recommends the 5 tablespoon to 1 cup formula also recommends a ratio of 15% by weight.
Liquid hide glue formulas using salt seem to be a little harder to find than formulas using urea. In my research I came across a lot of advice that suggested using salt in weight ratios similar to urea. One formula suggested that the best ratio for either salt or urea was 15% of the weight of the dry hide glue. However, there were also several formulas that conflicted with the recommended ratios of 5%-30% by weight. One of the sources that recommended 5 tablespoons of urea to 1 cup of dry hide glue also recommended 3 tablespoons of table salt to 1 cup of dry hide glue. 3 tablespoons of table salt weighs approximately 2.1 ounces. So this formula uses a ratio of about 38% salt to dry hide glue. Don William’s formula had the highest ratio of salt to hide glue. Don William’s recommends using 1 part salt to 2 parts dry hide glue by weight, or a ratio of 50%.
I tried to find some urea locally since it is what Patrick Edward’s uses. In the past I have seen 5 lb. bags of Espoma brand urea at the big box stores. However, the gardening section is still a little slow this time of year – I’ll give it a few more months. In the absence of a local source of urea, I decided to try to make liquid hide glue using one of the table salt formulas. I decided to split the difference between the high end and low end of the recommended formulas and use a ratio of about 30% salt. I added the salt to a fresh batch of hide glue I had made the day before and refrigerated over night. I reheated the hide glue and slowly stirred in the salt a little at a time. I then allowed the whole thing to cook for an hour or two. I added a little water part way through to make up for the evaporation caused by the heating.
I have done a few experiments with this batch of liquid hide glue and it seems to work well. The glue has plenty of liquidity. I placed it on a piece of wood in my shop to see if the glue would gel. After 5 to 10 minutes the glue was still slippery and there were no clumps. I glued up an edge joint using the liquid hide glue and the next day I hit it with a hammer. The wood failed before the glue joint failed. I have been storing the glue in the fridge in a small 8 ounce squeeze bottle that I purchased from Rockler. The glue is pretty solid when it is removed from the fridge, but it quickly liquefies when placed in warm water for a few minutes.
My biggest concern is how long it will last. Supposedly the more urea/salt you add the more you will reduce the bonding strength of the glue. In small amounts this isn’t an issue since the glue is plenty strong to start. However, the anti-gelling agents in liquid hide glue will continue reduce the strength of the glue over time which is one of the reasons that liquid hide glue expires. In my next experiment I will probably try only 10-15% salt by weight to see if that is sufficient to provide the desirable characteristics of liquid hide glue. I assume that the 30% ratio I used in my experiment is a little high. However, I’m not overly worried about the shelf life of my glue since I only made about a 2 ounce batch. It is simple enough to make that it is better to make it in small quantities as needed to ensure that you always have the freshest glue – and hence the strongest bond possible.
If anyone else has done some experimentation with liquid hide glue formulas please leave a comment describing your formula and your results.