Yeah, where is Robert by the way? Hope he didn’t have an accident or something.
I’m going to go contrary territory and say that the 60 watt bulb next to the block with the hood closed could have helped enough to make a difference. The radiator might still freeze up a bit, but the engine is the costliest part between it and the radiator, and the main one to worry about. It could have helped more if a covering like a blanket or plastic sheeting were also then held over whole front end, especially to block air from getting through the radiator.
You aren’t going to find any 100 watt or 75 watt or even real 60 watt bulbs around here anymore. They have been outlawed. The 60’s are all 60 equivalent which are more like 45’s. Of course I have my own over-stock of them, which are turning out to be obsolete since I swapped a lot out to LEDs.
Actually a bulb would be better near the battery, and I grew up in duluth MN and the 100 watt bulb was what we used in the 50’s. If you are looking for the best a block heater is primo, and they also sell battery jacket heaters.
Good point Bing.
They’re only available on the black market now… I’ll sell you one for $XX…
(for the feds listening in, I’m only kidding).
Reminds me of my dads 58 pontiac…Car ran great until the temp went down to 0 degrees…he even had a spare fully charged 12v battery he kept in the warm house and used jumpers to get it to start. One day it refused to start, ran both batteries down. he got his neighbor who had a 52 chevy and had to be push started. Thats when you could push start automatics. He finally got rid of that car and bought a 67 chevy. That little docile 283 would start right up at 10 below on its second revolution… He missed the power that the Pontiac had with the 370 4bbl. I was only a tyke then so do not remember much. I know he had the Ole pontiac looked at many times and they could not find anything wrong with the carb.
Were you putting in 50/50 mix? What you need to do is put in a gallon of full strength.
You can still get real 300-watt incandescent bulbs which are mogul base. How do I know? I have 2 old torchiere lamps that use 100-200-300 watt 3-way bulbs. In the summer I swap them for compact flourescents with an adapter, but in the winter, it’s actually nice to have the additional heat in the room 
But in the OP’s case I’d get the coolant taken care of—drain and refill with 50/50 mix. Maybe even a flush if it’s not been done in a long time. Agreed with what others have said that -10 isn’t really that cold, but I’d still do it. While you’re doing the coolant change, think about getting some synthetic oil in your engine too–your engine will be a lot happier on that first cold start of the day when temps are this low.
If you go to Amazon and type in “block heater”, you will find a lot safer and more effective alternatives to an ineffectual light bulb. Even a magnetic heater would likely help as some of the heat will be conducted through the block, though the majority of your coolant would not be affected by it. But -10 isn’t really cold enough to worry about getting one of these IMHO.
I used to live in an area that routinely saw -10 or -20F temps. A 60W light bulb would just be wasting electricity. There is no way you could adequately insulate the engine to retain the heat. Too many things in the way to fully enclose it. Might as well try peeing on the sun to put it out…
Nobody there did anything to heat the motor until it dropped to -25F or so, then it became more uncertain if your car would actually start. -10F is nothing if you have a good battery and the ignition system is in good shape.
I don’t think the OP is worried about starting, he is worried about cracking the engine or radiator, otherwise he would not have mentioned what his anti-freeze tested to. He should increase the strength of his anti-freeze, he is going to need to do it before winter gets into full swing in Denver.
As others have said above, OP should be adding concentrated antifreeze, not 50/50 mix. If his fluid is testing to only -10F freezing point, it is seriously diluted. Even 50/50 Prestone mix has a freezing temp of -34. And a 70/30 mix has a freezing temp around -80F.
http://prestone.com/products/antifreeze_coolant/product_list