Just want to thank all those that replied with the above helpful and detailed post to my problem. All of them have helped me to work towards solving this problem and I have learned many more things that I didn’t expect. It is kinda funny how you get set in your ways of casting and do not expand much out of that bubble as long as things are working fairly well.
If anything, this experience has taught me that I still need to pay close attention to all steps and not cut corners or take shortcuts. After years of casting you seem to start finding ways to make it a little faster, some of these might be good but some can cause bigger problems when one aspect of the process fails only slightly. I am going to try and do a complete post of what worked in this case as I am sure it will help others who might come across this later down the line with their own problems.
As of now, I am also going with the most likely cause being an investment issue
(possible contaminated investment or even contaminated water).
I think it also could have been compounded by the shape of the piece and the spruing configuration I decided on.
Good news is I cast two copies last night and the last one was a complete success! (wipes sweat away from brow)
The other one would have also worked if I had not reduced my casting temperatures at first thinking the original problem might have been something to do with to much heat . This reduced flask heat caused the piece to not fill completely.
This was at 1:30 in the morning so i was pretty nervous casting the final one thinking well this is it. It better work or im tossing this design out the window
So this is what I did differently for the casting that worked:
Wax #1- I sprued one skull the same as the last two fails except I added a sprue to the forehead where the problem blister was on the last pieces.
Changed: Wax #2 - I actually turned the wax 90 degrees and sprued main sprue to the head for a different configuration.
Changed: One thing I have never added is a venting sprue but I added this to both waxes. I have no idea if this even helps with vacuum casting but thought it might not hurt to try.
Changed: I went down to a local jewelry shop and purchased some of their investment in case mine was bad (they use the same investment I do which is Kerr Satin cast 20)
Changed: I mixed using distilled water and will do that from now on (I have always used filtered water from a nice sink filtration system I have. But I have gone several months past the filter change and wanted to take that variable out of the equation)
I invested the night before and then stored overnight in my steam dewaxing pot like I have always done (lets it set up nicely but does not let it dry out to much in my opinion)
Changed: Installed a temperature gauge in the top of my dewaxing pot and monitored the steam dewaxing as to not go much over 200 to 225 degrees while dewaxing for the usual 45 min to 1 hr.
Pretty much stuck to a good long and slow burnout schedule holding at 300, 700, 900, 1300 then let the flask soak at casting temperature.
Changed: The one thing I did differently was to put the dewaxed flask in a preheated kiln at 200 degrees instead of my usual 300 and slowly come up to 300. I also did little stops and slowly came up through the temps from 300 to 400, 500 and 700. Might have been overkill but I really wanted this casting to work.
I usually soak my flask at 1100 before casting and that has always worked but decided to drop the flask temp to 1000 with the first casting to see what would happen. So I let the 3.5" flask soak at 1000 for 1 hr and cast with metal at 1780 and while the pour looked good the skull had solidified before a full fill around the chin and lower cross bone.
I then increased the tempo to 1100 and let the 4" flask hold there for another full hour. I poured the metal at 1780 again and everything worked great.
So in conclusion it could be several factors that contributed, from spruing to dewaxing to quickly, but it seems the biggest change I made to cause the spalling / blister effect to go away was to change my investment and use distilled water.
Guess one thing I could do in the next few weeks is do two small casting, one with my investment and filtered water and one with the new investment and distilled water and compare the castings afterwards.
Thanks again for all the great help and I am glad to have become a part of this great jewelry resource i’m sure I will be visiting often!
Below are some pics of the way I sprued the final wax and how it turned out.
At the very bottom I also uploaded a pic of the other one from last night that did not completely fill.
For those that think it could have been mostly or solely due to the original way i sprued the first three (2 or which had the defect) here is a pic of the 3rd casting that was pretty much sprued the same way as the others that failed with the exception of a sprue to the forehead. As you can see the pour was actually very good with the exception of solidifying too quickly (this was my fault for lowering the flask temp to much). This again is not an ideal spruing but i do feel if the flask temp was up it would have been a good piece. So again it seems to point more towards investment breakdown for the original problem.