How can I keyframe the gain of a reverb to make the audio ramp from dry to wet and back?

I've got a reverb on the effects rack of track 1, and I want to be able to ramp the reverb gain using an envelope. I can't find any way to do this, other than duplicate the track so I have one with reverb and one without, and mix between the two. Surely there's a more elegant way?

I've experimented with all the Rack envelopes, and none do what I need. The one that comes closest is Rack Power, but that's basically a hard on/off control for rack effects, so it won't allow for gradually ramping the reverb gain up and down.

For absolute clarity, I need to be able to make the track ramp from dry to wet and back again at various point defined by keyframes on an envelope. Ideally I want the envelope to be *only* affecting the gain of the reverb, *not* the gain of the dry source track.

What's the best way to achieve what I want? On an analogue mixing desk (yes, I'm that old) I'd do it either by turning the reverb send level up and down, or using a pre-fader send to the reverb unit and bringing up the reverb return on its own fader.