Step 3: Mix up the Concrete
Mortar mix when cured in a glass mold like we are using gives a very nice gloss surface. The lower the amount of water you use, the smoother and like glass the surface is, and the stronger the resulting cured concrete is. However, the lower the amount of water you use, the harder it is to have it fill in the gaps on the sides and it leaves lots of holes and divots. It's getting the mix liquid enough to spread out but not too liquid that it looses it's strength is one of the issues on the water to dry mix ratio. Finding the correct compromise between these two issues is really a matter of practice and personal taste, I would suggest you play around with it in multiple bulbs if you are interested in getting the best result you can.
The ratio I've found works well is about 1.25 cups of the mortar mix and a hair under four tablespoons of water. So measure out a little under four tablespoons of water and put that into your plastic tub. This is more mortar mix than you need to fill a lightbulb, but there is always some spillage and trying to reduce the amount means even more accuracy on the water measurement. This is a good place to start.
Slowly mix in the mortar mix a little at a time. Let a little bit get wet, then a little more, then a little more while stiring the hole thing. It's a bit like making biscuit dough at this level, but your pouring the dry into the wet instead of the other way around. The consistency should be good enough that the mortar mix wants to stick together in one large clump, but it isn't sopping wet. If you feel you need to add more water or mortar mix to get it correct, then go for it. Just do it a little bit at a time. A small amount of either material make a large impact at this point.
Once it's at a consistency you like, keep stirring nice and slow for a few minutes. You want everything to be throughly wetted as much as possible.
Important! At this moment the clock starts and you have 30 to 45 minutes to finish the lightbulb before the mortar mix starts to harden up.
Once you have throughly mixed up the mortar mix, bring out the lightbulb and start filling it up.