I bought a bottle of Titebond Liquid Hide Glue and a can of Zinsser Sealcoat a couple weeks ago. According to the date codes both were manufactured within the past three months. I applied the shellac straight out of the can without thinning and after two days it's still soft and easily dented with a fingernail. Same goes for the hide glue. Still gummy after two days. Is it just coincidence that I got two bad containers of stuff at the same time or could something else cause this? Outside humidity has been high here lately (60%-85%). I've heard of humidity causing extended drying times but never this long.
The way liquid hide glue goes "bad" is by break down of the protein which leads to a brittle cured glue. If your test specimen is not hard it has not lost all the water.
The shellac has a slow drying solvent in addition to alcohol. Probably there to aid brushing. It takes a while to be rid of it.
You haven't given us a lot of information as to your location, so this is speculative to a certain extent. Hide glue and shellac are both subject to degradation due to high temperatures. There have been unprecedented heat waves in many parts of the world this summer due to the effects of anthropogenic climate change and significant supply chain disruptions due to the effects of the coronavirus pandemic, which could result in significant damage to these woodworking products anywhere between their manufacturers and your shop refrigerator. Since both cure by evaporation of solvents, the high humidity could have something to do with it, too, but in the case of shellac, that is usually evidenced by a blush in the finish.
Paint a think coat of each on a piece of glass. Each should be non-tacky in a few hours, no more than 12 probably given what you say about humidity. Thick coats and beads of glue squeeze out can confuse matters (and I use neither of these products personally, though I do use both hot hide glue and button lac routinely).
I've had button lac on a gunstock take weeks to fully dry - it was dry to touch but stuck to the fabric of a soft case. Nice thing about shellac is the fix was trivially easy (I don't normally use shellac for this, it was the period appropriate finish so I used it for the right color finish).
I've applied tons of shellac, I mostly use zinnser universal dewaxed. My understanding is that the date codes were no longer being put on the cans so I'm surprised you were able to figure that ourt I do agree with the suggestion that you put a drop on a piece of glass and see how it dries. First, regardless of the humidity this should not be "soft" after two days. How many coats have you applied, how did you apply them-brush, rag, were they heavy coats? Normally I get three coats on in one day using a fine badger brush and in 1 lb cuts. The wax content in the version you are using can make the finish a little "soft" but not meaningfully so, but this is why I use dewaxed. I would suggest giving it another day or two and see how it feels. If there is any doubt sand it back with 220 to a point where the finish is more firm. To be sure, get a fresh can, the waxed versions give an amber tone if you don't mind that and the dewaxed version makes it a universal sealer.
Thanks for the responses. I moved the piece with shellac under a different light source and found some clouding in the finish so humidity seems to be playing a part. I may have also applied it thicker than normal giving me inconsistent results. As far as the hide glue goes, I made a new test piece by spreading some on a piece of oak and it dried just fine within a few hours. I also moved the original gummy test piece into an air conditioned area and it hardened up well. I've used both of these things before without any problems but apparently not in this humidity. So the solution is simply being more mindful of the weather conditions and a bit more patience next time.
I was spraying shellac yesterday, with a dew point of 71°F. Normally I can go back and sand off the nibs in an hour. Yesterday it took 3 hours and then was just barely hard enough.