Using Object Oriented Design with Ruby, how to show transfer of ownership?

Say we have the following classes: Bakery, Equipment, and Batch. Bakery is composed of Equipment like so:

class Bakery
  attr_accessor :equipment
end

Equipment looks like this:

class Equipment
  attr_accessor :name, :quantity
end

So to represent a bakery with two ovens we can do this:

ovens = Equipment.new
ovens.quantity = 2
bakery = Bakery.new
bakery.equipment = [ovens]

Now the Batch class represents one batch of baked goods. It is itself composed of a Formula with several Step objects. The Formula class contains the order of steps and timing information so that Batch knows when some equipment needs to be used. (I'm leaving out the Formula and Batch code for brevity. Full code is available here and here.)

My question is how do I use OOD to temporarily transfer ownership of some equipment (say, 1 oven) from bakery to batch. bakery holds an instance of Equipment that represents two ovens. One of those ovens is going to be temporarily occupied by a batch process. Should I create a new instance of Equipment and pass it to the batch while decrementing the quantity of ovens? Or should I not bother with the quantity attribute and simply create one instance of Equipment for each oven in the bakery? In that case I could simply pop the last oven off the bakery.equipment array and push it onto batch.equipment until batch is finished. One business logic requirement is to be able to query future availability of bakery.equipment. So there needs to be some distinction between the current state (i.e. how many ovens are occupied) and future states (i.e. how many ovens will be occupied at 9:00 PM).