Kiln optimization problem
Either there is an additional constraint, either this problem is far more simpler than task scheduling.

Well you have to identify the limiting criterium of your problem which is 50 molds maximum in the kiln.

Let's call S = A+B+C = 536, the total number of molds. So You cannot do better than 11 firings (11*50=550).

First compute the euclidean division of the required elements to get the quotient and the remainder, that will be the constant molds and the variable molds to use.

- x, a = A/11 = 9, 3
- y, b = B/11 = 33, 1
- z, c = C/11 = 6, 4


So the constant molds are (x, y, z) = (9, 33, 6), that is to say 48 molds. 

You now have 11 firings on the 2 (50-48) variables molds to complete the (a, b, c) = (3, 1, 4) remaining elements.
Let's say we want to minimize the number of molds to manipulate. Then just take one extra mold of each A, B, C to have a total of 51 molds.