Ted's glue experiments

[John Hartman]

Dear Ted,

Good to hear from you. I have been following the discussion of hide glue 
wood joinery techniques with interest. I have not done the extensive 
experimentation you have and I applaud you for going the extra distance 
in your work. I have relied on professional advise in the use of 
adhesives. When I started using hide glue for wood joints I received 
detailed information from Mr. Bjorn of Bjorn Industries. Mr. Bjorn is a 
chemist formerly with the Cooper Glue company. He gladly shared a great 
deal of information about the proper use of hide glue in furniture 
making. I know most of this information will not be new to you but I 
thought it would be useful to add another dimension to the discussion.

Hide glue comes in many grades. There have been a number of systems for 
defining hide glue grades. The one most commonly used is the Cooper grade 
system also known as the gram strength system. Hide glue samples are 
carefully mixed with water in exact proportions. The samples are heated 
to 140 f. and the viscosity is measured. Low viscosity indicates lower 
strength and higher indicates higher strength. 80 gram glue is the lowest 
grade while500 gram is the highest grade. Low grade glues will gel slower 
than high grade glue. I am not sure how the different grades are made but 
they are produced deliberately for different applications. Lower grade 
glues are not a mistake but are needed in some applications. The highest 
grade 500 gram is used for a number of special applications including 
glass etching. It gels in just seconds and has no uses in wood working. I 
mention this only because I have heard some piano technicians claim they 
only use the highest grade of glue in their work. Most hide glue used for 
woodworking joinery is in the middle range. 

Each grade of glue has a specific amount of water to be mixed with it in 
order to achieve maximum strength. Low grade glues need less water than 
high grade glues. Mr. Bjorn suggested I use 251 gram glue. It is easy to 
remember the proportions of glue to water for this grade (for woodworking 
joinery). Two portions of water for one portion of glue. So if I am 
making glue with 100 grams of glue I carefully measure 200 grams of 
water. I use a gram scale to measure this. If you ad more water the glue 
will gel slower but the strength will be less. If less water is used the 
gel time will be shorter. 

I also received sound information on the use of urea to extend the gel 
time. Apparently this technique has been used successfully in industry 
for quit some time. Adding urea does weaken the glue but not to a degree 
that will weaken the resultant glue bond if the amount of urea added is 
bellow 30% of the dry glue used in the mixture. I have found it essential 
to add 15-20% urea for glue ups in the soundboard building process. It is 
a good safety measure since the two mating surfaces of wood must be 
brought together while the glue is still in a liquid state. Even with 
urea added one must work very quickly to glue up something large like a 
soundboard panel for example.  

I have been using this grade of glue and mixing as above for over seven 
years with virtually 100% success. The glue is mixed fresh for each days 
work. Most surfaces are toothed (except panel edges) and heated for a 
short time with a heat gun. The wood is not heated in a chamber until it 
is heated through. This would lower the moisture content too much. I 
mention this because some people have the mistaken idea that conditioning 
chambers are used in this way. Probably because they are called "hot 
boxes". I do use a "hot box" but only for controlling the moisture 
content of the soundboard. Glue is applied to one surface only and 
clamping pressure is applied immediately. I test most of the joints with 
a .005" feeler gauge.  I rarely find that the gauge will slip into the 
closed joint but if in does I apply more pressure and use the heat gun to 
get the glue flowing. In some applications the squeeze out is cleaned off 
immediately with hot water.

I use hide glue for a number of wood joinery applications including, 
soundboard panels, ribs to soundboard, bridge to sound board, soundboard 
to inner rim and veneering case parts. In fact I once used hide glue to 
glue up a 1865 Steinway grand wrest plank made as an exact duplicate. I 
think the problems industry experiences are due to mishandling and 
general ignorance of how hide glue works. Clearly hide glue is not 
foolproof but in the hands of experienced artisans in a small scale shop 
it is the first choice in adhesives. 

John Hartman RPT