Working with traditional animal glue is an extremely simple operation. Don't overthink the process and you will be fine!
Your three common Gram Strengths or "Bloom Strengths" are as follows:
192 Gram Strength Hide Glue:
- Long open time
- Strong bond
- Also known as "Marqueter's Grade" or "Veneer Grade".
- Medium initial tack
- The ultimate glue for marquetry, parquetry and all veneering work.
251 Gram Strength Hide Glue:
- Medium open time
- Very strong bond
- Also affectionately known as "Cabinetmaker's Grade".
- Fast initial tack
- The best general joinery and cabinetmaking grade hide glue.
315 Gram Strength Hide Glue:
- Short open time
- Incredibly strong bond
- Also known as "Luthier's Grade"
- Very fast initial tack
- Used for all Lutherie applications and where you need an exceptionally strong bond with very quick tack.
In it's dry granular form, hide glue has a completely unlimited shelf life when stored in a dry container and kept away from heat and moisture. (I have cooked up hide glue from 38 year old granules and it worked perfectly).
To prepare the glue for use, you simply add cold water to the granules and let it sit overnight. It is not critical as to how much water you add, so long as the granulated glue is completely covered.
If you feel the need for precision and wish to mix by weight, use 1.8 parts water to 1 part granulated glue.
Once all the water has been absorbed, your hide glue will look like a gelatinous mush and is ready to cook.
Tobias' Tip: Lay your hands on a few ice block trays. Mix up sufficient glue to fill all the cubicles. Let them sit overnight to gel and then pop the trays in the freezer. Once the block are properly frozen, pop them out into a suitable container and keep them in the freezer. Repeat the process a few times and you will have enough "Ready To Cook" Hide Glue for a few months!
(Be sure to mark the container clearly, as hide glue tastes really odd in a gin & tonic!) Making and freezing the blocks eliminates the overnight wait completely. When you want to cook up some glue, simply grab grab a few blocks and head for your workshop!
Also, when you have finish with your glue and don't want to waste it, simply pour it back into the ice trays, let it cool and freeze it again into blocks for later use!
To cook up the glue you need a double boiler (that kitchen thingy that granny used to heat milk in), or a traditional cast iron double glue pot, or you can use my cheap favourite,...a baby bottle warmer. I use my traditional glue pots only when I am doing period craft demonstrations at the museum.
A Double Boiler that was commonly available in SA, made by Hart.
Tobias' Tip: For daily use in my workshop, I rely on a cheap thermostatically controlled baby bottle warmer from Clicks and a glass jar which fits inside allowing a small space around the jar for the water.
Whichever system you prefer, you will need a digital cooking thermometer, which is available from Toolcraft. This is by far the easiest and most accurate way to monitor your glue temperature.
You need to bring the glue up to a reasonably constant temperature of 145º Fahrenheit (63º Centigrade). A couple of degrees either way is not a train smash.
If for some reason, your glue goes dramatically over this standard temperature, throw it away, as you have more than likely broken down the protein chains and rendered it useless.
Keep your glue pot at a constant heat by adjusting the heat control knob. Don't believe the temperature marks on the baby bottle warmer, rather place your trust in your digital thermometer.
Tobias' Tip: Cover your jar with a piece of aluminium foil (shiny side down) and fold the edges so that it forms a lid covering the entire bottle warmer. This will keep most of the moisture in, reducing the need to monitor and top up the water jacket. Just poke a hole with the digital cooking thermometer to go through the foil lid and you are all set.
Generally and historically speaking, traditional woodworkers use "subjective testing" to monitor the glue as it cooks. The odour of the glue should be quite pleasant, if the glue is good.
Don't believe people who tell you that hide glue always stinks... There are only two reasons why a pot of hide glue can smell bad. Firstly, if it has been overheated and has therefore become useless or secondly, when it has been damaged by mould. This happens when you leave mixed glue unheated for extended periods in the jar in the open at room temperature and just like any food or animal based product it will go off and start to smell. (Have you ever smelled a bowl of porridge that has been standing out in the kitchen for a few days?...the same thing applies here).
The viscosity of you glue mix is easily measured by lifting your glue brush about 300mm above the glue jar and letting the hot glue drip back into the jar. It should be thin and liquid with no clumps.
You can easily test the strength of you glue by putting a small amount between your finger and thumb and rubbing them together until the glue cools. You then measure the strength by slowly pulling your thumb and finger apart to create a gap of about 4-5cm, and observe the protein strands which will look like very fine spider webs. The longer the strands...the stronger the glue. It's that simple!
When it has been freshly cooked, the colour of protein glue is a light amber shade, and it will continue to darken the longer it cooks. As long as you do not overheat your glue batch, it will remain immensely strong.
Tobias' Tip: If you have any cooked glue left over, just put the jar in the fridge overnight and you can reheat it and continue to use it the following day.
Animal glues are the only easily reversible glues available to woodworkers.
All modern synthetic glues convert from one state to another state by using a catalyst. Once converted, the adhesives are for the most part, impossible to undo. Since protein glue reacts to heat and moisture, they can easily be converted from liquid to solid and back again, even after a century or more!
This is the primary reason as to why hide glue is used continuously in the field of antique furniture restoration. The existing glue can be softened and cleaned off with warm water and a new application of hot hide glue will completely bond with the previous glue and the wood.
Glue reversibility is also of essential importance when working with veneers, inlays and marquetry surfaces. When you are building up a pattern or picture of wood pieces on your substrate, it becomes necessary to glue and unglue, and shift pieces into place as you progress, only hide glue allows you to do this competently.
Hide glue also allows easy repair and replacement of damaged veneers on furniture, by using heat and moisture.
Also, don't forget the fact that hide glue does not exhibit the dreaded "creep", that synthetic adhesives do when clamping boards together, due to it's high initial tack.
Above is my standard glue line-up. Left to Right: Brooklyn Tool & Craft 192g Marquetry Grade Hide Glue, Click Baby Bootle Warmer with Consol Jar, digital thermometer and tin foil hat, Brooklyn Tool & Craft 251g Cabinetmaker's Grade Hide Glue, Old Brown Glue Liquid Hide Glue, Titebond II and Titebond II both in GluBots!
Another of the wonderful attributes of hide glue is that it's incredibly easy to clean it off your wood surfaces, either with a little water, by sanding or using a cabinet scraper.
Most importantly, hide glue is not affected by dyes, stains, solvents, oils or varnishes and does not show up underneath your finishes!
In Part #3 of this series, I will talk about Liquid Hide Glue...
Until then, let's make shavings, not dust!
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- Tags: 192 Gram Strength, 251 Gram Strength, Brooklyn Tool & Craft, BT&C, Hide Glue, Hide Glue reversibility, Hot Glue, Scotch Glue