A reactive glaze is formulated to move, break and vary in the firing rather than to lie flat and even. That is what makes it attractive and it is also what makes it unpredictable: thickness, edges, the body underneath and the position in the kiln all change the result, sometimes dramatically.
What is actually happening
Reactive glazes usually contain enough flux to become quite fluid at temperature, plus materials such as rutile, iron or titanium that separate out as the glaze cools. The fluidity lets the glaze thin over edges and pool in hollows, and the separation gives the broken, variegated colour. Both effects depend on how much glaze there is and on how the kiln cools.
Why the same glaze gives different pots
Thickness is the biggest variable: a dipped rim carries less glaze than a pooled interior, and the colour differs accordingly. The clay body matters too, because iron in the body feeds the reaction. And kiln position matters, because cooling rate varies across a chamber. None of that is a fault; it is what the product is sold for.
Keeping it off the shelf
Fluid glazes run, and a reactive glaze that runs takes the kiln shelf with it. Leave more bare clay at the foot than you would with a stable glaze, use a well-washed shelf, and consider firing test pieces on a waster tile until you know how a particular glaze behaves on your body at your temperature. This is the most expensive small mistake in the hobby and it is entirely preventable.
Getting a repeatable result
Standardise what you can. One body, one dipping time, one kiln schedule, one place in the kiln for the pieces that matter. Then the variation that remains is the pleasant kind. Potters who sell reactive-glazed work usually describe each piece as unique, which is honest marketing and also an accurate description of the process.
Questions people ask about reactive glaze
Why do my reactive glaze pots all look different?
Thickness, body and kiln position all change the result. That variation is what the glaze is formulated to produce.
Will a reactive glaze run onto the shelf?
It can, because these glazes are fluid at temperature. Leave more bare clay at the foot and use a washed shelf.
Can I make a reactive glaze more predictable?
Standardise body, thickness, schedule and kiln position. The remaining variation is the effect you bought the glaze for.