How to routinely obtain consistently superb metal castings: Okay, you've recovered the metal castings from the investment, and they appear exactly as the wax patterns that you formed during wax-sculpting and cusp/fossa analysis. You inspect for  micro-nodules using magnification and remove them carefully before gently test fitting the metal to the dies..but to your absolute horror - they don't fit. They're too tight..or too loose. Why did this happen, and what do we do now? If you have any sense of pride in your work, you'll throw them into the scrapbox now before you compromise the master dies, and solve this problem right now. I've worked in well-equipped labs where the only thing missing was an alter for technicians to pray that the castings would be complete and fit properly..as if there were no way to consistently control the expansion/contraction of the materials and ensure complete and flawless castings..but this just isn't true. Predictable results may always be expected -  IF - you resolve to take the time and steps necessary to achieve excellence. From spruing and pattern placement within the properly lined ring, to proper mixing ratios, consideration of temperature and humidity of the environment, to proper surface tension reduction and proper 3 stage burnout of the patterns, to proper oxygen/fuel gas mixture and torch tip placement..you can  see that it isn't luck how some technicians ALWAYS get positive results  while others get ulcers.
A crown or bridge abutment casting should be free of bubbles if you invest properly. Most folks prefer to use a surface tension reducer..a few use plain tap water. I use Smoothex from the Whip Mix Corp. I've used them all at one time or another. The real trick is to observe the investment material flowing into each wax pattern. To assist in this regard, I prefer a light colored material such as Fast Fire 15 or High Temp..I still have and use black investments such as Ceramigold,..but prefer the white ones to better see it flow into the patterns. You will sometimes find micro-nodules adhering to the castings. Be absolutely certain you remove these nodules and any "flash" resulting from transparant wax overextending the finish line. Do this before you test fit the casting. These nodules are best removed under high-magnification with a high speed turbine and carbide and diamond cutting instruments. I use new SS White #34  inverted cones & 33.5 round burs to accomplish this task. Before the test fit, I use a white AlO3 barrel shaped SHP stone to "shine" the margin. I always place a wax margin on my castings for assurance of a complete casting and intimate marginal fit. When you've done these steps, the casting may then be introduced to the die. If you've done your job properly, it will fit smoothly, seating all the way down on the master die with desired slight frictional resistance. If it rocks, the wax patterns were probably distorted, and if it won't seat - your investment liquid ratios are suspect. In either case, throw the casting in the scrap box and re-wax a brand new one right now, making sure you draw it on & off the die several times. It should exhibit that slight frictional resistance and fit perfectly 360 degrees under magnification. Resist the urge to attempt to fit a bad casting to a pristine die. It never works, and although some dentists will fill it with cement, it's bad technique. Don't be a hack. You probably won't get rich, but you can take pride in knowing you do it the way it should be done. I do a finished wax-up. That means I have little to do in metal other than prepare the porcelain bearing surfaces with non-contaminating AlO3 stones, remove the metal margin and define the finish lines with a sharp SHP carbide cutter. This is done with the casting off the die. The dies must be kept pristine until the crown is finished and is ready for polishing. I polish my work on the dies to protect the margins.  When finishing a labial or buccal margin or collar, check for metal thickness of the casting and maintain at least .2mm thickness with base alloys, and at least .3mm with noble & high-noble alloys. Your occlusal fossa can be .2mm if you wish. I've never seen an occlusal fossa fail at those dimensions. Marginal fractures however, can readily occur if the metal at the cervical margin is too thin. Metal bends - Porcelain  does not - it fractures.
When the castings are verified in the lab to fit the die(s)exactly, then you must prepare the static casts to receive them. Do this by attempting to trim the die(s) on this 2nd cast as if they were the original dies. This step won't be as accurate as the master die, since the die(s) are not removable, but make the effort. The closer you get, the more stable the fit and therefore the proximal contacts will be more exacting. Don't just trim the margin away with a "jungle" bur..you can establish solid buccal & lingual marginal fits with no rocking. When the fits are established on the working and static casts, the time has come to clean all the casts and dies, and the metal castings must be sandblasted. I use 50 Micron pure AlO3 abrasive blasting compound @ 50 psi. Of course you must have adequate suction and always wear a protective mask and safety glasses. Don't be a moron. Place the blasted castings on a 2x2 gauze pad in a beaker of distilled H2O and ultrasonic rinse them for 5 mins.   The castings must never again be handled with bare hands. Always pick them up with clean hemostats or forceps. Blot them dry w/ tissue and place on saggar trays for oxide development in the vacuum furnace. Follow the alloy manufacturer's recommended instructions. When cooled, you may begin the porcelain applique.	These steps are required..don't try to cut corners or fudge them. Have some pride in your work..you're in the business of making people look good and enjoy nourishing foods that keep them alive and well, not to mention the emotional benefits they derive from conversing and smiling naturally without fear or embarassment. Be a dental technologist instead of a "Lab crab" or a "plaster monkey"..