Ok, here is the castable walkthrough I promised minus any pictures. If anyone would like a picture of something specific ask and I will post it. Something came up and I ended up short on time tonight.
I started out by finding a suitable dense castable refractory that I could source locally. After a few phone calls it became clear in my area my only real choice for small quantity cash and carry was Harbison Walker. You can find your local brach here:http://www.hwr.com/
To be honest I found their staff to be of little help, but luckily I did my research and seemed to know their products better then they did. What you are looking for is a Dense Castable, Dense being the key word because there are also insulating castables which are not as strong. All of their products have a duty rating of at least 2550F so my advice is to got for the cheapest one they have since it is vast overkill anyway. In my case they have KS-4V plus in stock for $28.50/55lbs. It is good to 2600F and has a yield of 125 lbs. per cubic foot. For my casting I needed about a bag and a half, so I bought two. I have the full data sheet on this product if anyone is interested.
Harbison Walker was unable to tell me if I needed reenforcement for my purpose, but they did tell me it can be used with this product. I decided that it would give me piece of mind to have it in there being this is a structural piece that will have the lateral load of the low dome on it. My local branch did not stock stainless needled, and instead needed to special order them from chicago. That sounded like a bit of a hassle to me so I went a different route.
High Temp Inc. sells Stainless needles for refractory reenforcement on ebay in 10# quantities here:http://cgi.ebay.com/Refractory-Reinforcing-1-Stainless-Steel-Needles-10-/130337734449?_trksid=p5197.m7&_trkparms=algo%3DLVI%26itu%3DUCI%26otn%3D2%26po%3DLVI%26ps%3D63%26clkid%3D953544977152901738#ht_500wt_1156
In their ad they mention they will sell any quantity, and to call 1-800-325-2492. I called on lunch last thursday and ordered 2 pounds. They are in Oregon, I am in Michigan. Saturday they showed up in my mail in a flat rate box all for about $10 shipped. I can not speak highly enough about these guys! They suggest 3%-5% by weight and I fell in the middle of that range.
At this point it is time for a form to cast into. MY casting was fairly complex and being a sheetmetal worker, I built it from annodized aluminum. Obviously use materials and methods here that suit your skills and tools. The key part is to make a form that can be opened wide for removal. My form have 8 parts that could all be seperated and worked out nicely.
The KS-4V plus I used can be rammed into place or vibration cast. There are different amounts of water mixed for each. Vibration was highly advised by everyone I talked to, and I decided to build a small vibration table to achieve that. I basically followed the design in this video:
The only place I deviated was by not buying a pre-made vibrating motor and instead making my own from an old motor in my stock pile. To make the motor vibrate I simply installed an old pulley on it and used a metabo to cut away at one side of the pulley until I was happy with the vibration. I ended up cutting about half of it off.
Finally it was time to cast. I put the form on the vibration table and actually screwed it down to it for better results. I then lined the form with vegetable oil as a release agent. My form was pretty nonstick, if yours is not I was told motor oil and kerosene 50/50 is an great release agent. I then hand mixed the refractory and the needles dry with gloves on. Once I had a good mix I used a scale to measure out the water out to the amount the data sheet calls for and mixed it in. I then quickly shoveled the mix into the mold stopping every now and then to vibrate and ending it all with a fairly long vibration. The specs say to be careful not to overdo the vibration to the point of segregation so I was very cautious. I again followed the specs and waited 24 hours for a "hard set" to open the form, although I had a hard set after about 18 hours according to their test which was to strike it with a metal rod and listen for a distinct ring.
I think that about covers it for now.