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Old 26-07-09, 04:09 PM
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EllenT EllenT is offline
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Location: Bruges, Belgium
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Default Adhesion problems with gesso

Hi All,

Though I 've been making gesso panels for about 30 years (on and off) with reasonably good success, I've had difficulty with my batches within the last five years (since we moved to Brugge, could it be the tap water!?!).

So, I've had problems of adhesion, where the first coats were thin and things seemed to be going OK, but as the later coats developed, it was clear that the gesso was not building up and adhering in a uniform manner.
I've tried various things like rereading the instructions from Ralph Mayer or Reed Kay, internet research and buying new quantities of fresh glue, but a clear resolution eludes me. I bought a candy thermometer in order to monitor the temperature. Is it correct that 135 F (or 57.3 C) is the maximum temp for heating the glue?

In a recent batch, (what I considered to be) too much pigment came off while sanding, though the panel otherwise seemed OK. I then coated the final panels with a thin layer of glue size just in an effort to save my work and make it useable. Thus far it seems OK, but I would like to troubleshoot the problem once and for all. I'm hoping the combined experienced resources here can help?

Thus, the following questions: Does hide glue have a shelf life? What is it? Is one hide or animal better than another? Do they possess different heating threshholds? Could it be the tap water?, i.e. should I use distilled water? Anything else that I should take into account?

Thanks to any and all,
Ellen
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Old 26-07-09, 05:39 PM
Rosemary Rosemary is offline
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Default Things to consider

It does sound like your glue is not strong enough for some reason.

1. Do you weigh your dry glue or use a volumetric measure when making the solution? Weight is most accurate.

2. Do you test your glue for strength by putting a cup or so of it in the refrigerator until firm all the way through, then pulling a finger tip across the surface to make it crack?

Weak glue will not make a clean crack, but pulls apart in a messy, granular way. Strong glue will have as sharp a crack as if cut by a knife.

Glue that was improperly made or had a lot of bacterial growth or meat tenderizer enzyme in it will not be as strong as expected. Or glue that is in light fluffy granules as opposed to dense drops can not be accurately measured by volume but must be weighed.

If your water is acidic or basic, it could possibly cause the glue to hydrolyze into smaller, less strong, chains. Heating the glue too much does the same thing.

You could try making the glue with distilled water.

Good luck,
Rosemary
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Old 27-07-09, 06:38 AM
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EllenT EllenT is offline
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Default

Thanks, Rosemary.
Yes, I use a fairly precise weighing scale to measure my weights.
I have used the glue test in the past (refridgerating it, and testing it with my fingers to pull it apart), but I think I will need to return to regular use of such testing again until I am sure of all my variables.
Will try with batches of both tap and distilled water to see if there is a difference.
It seems possible that I have had a few batches of poor quality RS glue. Will order some new stuff from a reputable dealer - and test it too.
Does it have a shelf life?
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Old 27-07-09, 12:48 PM
dbclemons dbclemons is offline
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Default

Reportedly, dry RSG has an indefinite shelf life if kept in an airtight, dark, dry and cool location. One aspect is its gram strength. If that is too low (below 164gs) it might still be a proper glue, just weaker than what you need. It's not always declared on what you buy. If the glue takes longer to dry or has a strong odor you'll likely need to replace it. Also if the dry form is very fine powder it may need less water than if in flake form. I doubt changing the water will make much difference.
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Old 30-07-09, 04:40 PM
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EllenT EllenT is offline
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Default gram strength

Hi David,
Gram strength is a new one on me. I'll try to lookout for it. Dealing, as I often am with foreign language suppliers, it can be difficult to get the fine detail info - or even ask the right quesitons. Yet another reason why this site is so useful.
Cheers,
ET
http://www.ellen-trezevant.com
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