Potters & Sculptors - Making Rock from Mud
As you guessed each different raw material used as a glaze ingredient holds a very different amount of water. As a result the amount of water each glaze slurry recipe holds is different as well.
And further good or bad news, this can be adjusted and probably should be for each purpose.
For convenience I start with one pound of water for each pound of glaze. Four gallons of water weighs about 33 pounds, and glaze powder is less dense than water, so Amaco telling you to add enough water to 25 pounds of glaze to make 4 gallons of glaze is probably pretty close to 1 to 1 by weight.
Using metric grams to weigh a glaze is easier because one liter of water at room temperature weighs 1,000 grams which is one kilogram. So I use a plastic metric beaker for water rather than weighing it.
A few days after sieving the glaze, I toss-off any water standing on the top after the glaze settles. We can always thin down a portion of the glaze later if that's better for use. A very few glazes actually require more than a 1 to 1 ratio of water to glaze powder have a usable consistency. But most require somewhat less, a few far less.
Prepared dry glazes from Laguna mixed to instructions are typically very thin. The instructions tell you to add no bentonite for spraying, some bentonite for dipping, and more for brushing. In addition to bentonite we sometimes add Propylene glycol when we need to make the glaze brush more smoothly, while others use glycerine for this purpose and some use CMC gum which has the disadvantage of growing bacteria.
People who spray glazes typically adjust the amount of water until each glaze has a nearly identical viscosity as it is pumped through the sprayer, even though this requires a differing amount of water. A Ford Cup is typically used to measure this. Digitalfire Viscosity
Viscosity of some material changes significantly under pressure, such as Xanthan gum which helps a glaze become very liquid when flowing, yet instantly gels when applied to the bisque.
This is an interesting video, made by Tony Hansen of Digitalfire. But using an acid like vinegar to gel a glaze creates a problem with many glazes, as you'll discover when the carbonates present in many glazes immediately reacts and foam-up after adding the vinegar. Acids also tend to leach out sodium and potassium from raw materials which will hard-pan the glaze when they've been made soluble by the acid. We instead add Calcium Chloride crystals for this. Digitalfire - How to Gel a glaze
Some adjust the amount of water so each glaze has the exactly same density, an odd practice which has no practical benefit I'm aware of.
But many sensible people use a hydrometer to measure the density of each glaze when they get it to the where they want it and keep a record of it, as the "correct density" will be different for every glaze.
Amaco pint glazes all seem to have added a different amount of bentonite to make each different glaze into a nearly identical thick pudding which does not pour. I've never used Amaco dry glazes, so it's possible they've made the same adjustments to each glaze so every Amaco dry glaze becomes the same consistency, but I doubt it. Their entire line of glaze also have very compatible chemistry so can be mixed and layered without concern for bubbling or other odd interactions.
TMI . . . But there's one more thing to consider. The percentage of glaze weight lost as gas is different for each recipe. So a thick layer of some glazes fire thin, while a normal layer of some glazes is so much excess it runs badly. Amaco glazes tend to be heavy in silica as a filler to create a lot of glass, so there is less fired variation among their glazes - and why their glazes, apart from color, all look very similar.
Once you start to make or purchase glazes with a different look to Amaco's, the relative predictability you become accustomed to with Amaco is lost as you're now in the wider world of ceramics, so test tiles are essential.
I second what Norm has said, especially about the LOI. Depending on whats in a glaze and how much burns off, your glaze can either look very rich or pale and sickly. I usually start with 75-80 ml of water per 100 grams of glaze. That way if there isn't a lot of water absorbing chemicals in the glaze, it will be just about right and I usually have to add water to the others. One good thing about layering 2 glazes is you almost always get good coverage, but the bad thing is that quite often you get a lot of running, so you always have to test, test, test! jhp