Sweet Yellow Squash Pickles

3 to 3½ pounds tender young straight-neck yellow squash, trimmed, scrubbed, and sliced ¼ inch thick (you will need 12 cups sliced squash)
4 to 4½ pounds silverskin onions, peeled and thinly sliced (you’ll need 12 cups sliced onions)
½ cup pickling salt
6 cups (1½ quarts) crushed ice
3½ cups sugar
2 cups (1 pint) white (distilled) vinegar
2 cups (1 pint) cider vinegar
2 teaspoons mustard seeds
1¾ teaspoons celery seeds
1¾ teaspoons ground turmeric

Layer the sliced squash and onions in a very large nonreactive bowl, sprinkling each layer with salt. Pile the ice on top, set the bowl in the sink, and let stand for 3 hours.
Drain the squash and onions, transfer to a very large colander, and rinse under the cold tap. Drain well, then, using the bowl of a ladle, press out as much liquid as possible.
Wash and rinse 8 one-pint preserving jars and their closures and submerge in a large kettle of boiling water.
Bring the sugar, white and cider vinegars, mustard and celery seeds, and turmeric to a rolling boil in a large nonreactive kettle. Add the squash and onions and, stirring gently, return to the boil.
Lift the preserving jars from the boiling water one by one. Pack with pickles, making sure they are submerged in the pickling liquid and leaving ¼ inch head space at the top of the jar. Tip: To avoid spills, use a wide-mouth canning funnel. Run a thin-blade spatula around the inside of the jar to release air bubbles; wipe the jar rim with a clean, damp cloth, then screw on the closure. Repeat until all jars are filled.
Process the jars for 10 minutes in a boiling water bath (212° F.). Lift from the water bath; complete the seals, if necessary, by tightening the lids, then cool to room temperature.
Date and label each jar, then store on a cool, dark shelf several weeks before opening.