When I checked on it the next day to see if fermentation was occurring, I found a huge mess. Apparently the fermentation was so vigorous that juice and sediment bubbled out of the airlock, splattering all over the place. I took a little taste to see if there was any sugar left. It still tasted sweet, so I racked the cider into plastic soda bottles, and put them back to carbonate.
The next day the soda bottles were hard and swollen. The fermentation had not slowed at all, and the soda bottles had become high-pressure carbon dioxide bombs. The pressure was so high that the plastic bottles had deformed and stretched, so much so that the bottoms were pushed out and they couldn't stand up on the counter. I immediately threw the bottles into the refrigerator to kill the fermentation, and spent the next few days slowly releasing the pressure a little bit at a time.
Once I was convinced the bottles wouldn't explode when opened, I cautiously opened one in the sink. Psssssssssh! It bubbled and bubbled and bubbled. The carbon dioxide concentration in the liquid must have been at the maximum saturation. Once the effervescence calmed down a little bit, I took a test sip. Ugh.....It tasted like a permanent marker. The yeast had used up every last molecule of sugar in the juice, leaving behind a pretty terrible tasting, yet super-carbonated beverage. Too bad.
So what went wrong? My theory is that there were lots of wild yeasts and other microorganisms on the skins of the pears, and I probably could have made a cider without adding any yeast at all. Adding the champagne yeast to the already inoculated juice created a situation where there was a lot of yeast competing for relatively little sugar, and a microbial feeding frenzy ensued. The end result was the driest, worst tasting, most carbonated, and most alcoholic brew I have ever tasted or hope to see again. If I ever make cider this way again, I will either take great pains to carefully wash all the fruit before pressing to remove the wild yeasts, or will just let the wild yeasts do the work and not add any of my own. I'll probably just eat the pears instead of juicing them, and let St. Arnold take care of the brewing.