Piano keys need to be repaired or replaced from time to time. It's tempting to think any strong glue will do. Not so. Rather than thinking of a glue as being strong, it's better to think of it as being the appropriate type for your application. The glue must bond a porous material --- wood --- to a smooth material --- ivory. Piano keys require special glue.
PVC-E is speciality glue carried by piano shops specifically for gluing piano ivories. It's important to understand that, while PVC-E looks like other wood glues, it is not. Nor does it have the properties of other similar-looking glues, like Elmer's white paper glue. PVC-E is the industry standard for piano keys.
Porous and Non Materials
PVC-E excels at gluing dissimilar materials, whereas most glues tend to only join like-surfaces. It should be noted that PVC-E works as well on plastic or synthetic keys as real ivory keys. Modern pianos don't use ivory. But PVC-E is the correct glue for fastening synthetic keys, too. It's essentially the same bond: one smooth surface to porous wood.
PVC-E is a water soluble glue. You'll have plenty of working time to apply the glue, set the key and use a light clamp. Working time is ample, unlike clear, fast-drying adhesives. Curing time is at least 24 hours. Keys may appear to be set much sooner, but avoid contact before they are fully cured.
As with many other kinds of adhesive bonds, preparation is a very important component of a quality and lasting bond. The old wood should be cleaned of all excess glue. Some porousness may be lost because the old wood will absorb some of it. Just scrape the old glue off, so no excess interferes with a smooth, flat connection. The key should also be scraped clean of any old glue or debris. With clean preparation, PVC-E will hold the key in place for many years.