Tinderbox

I mentioned this last night in the Formula section in the Stars thread but thought I'd ask here too. I can't find any discussion on it but that's because I do not know the term for the phenomenon that is: 

A star which during drying, expands and cracks open. Usually in the latter stages of drying.

I have been working on a Sodium Nitrate based composition using fairly coarse MgAl. I used 1% boric acid due to the MgAl and Sodium Nitrate. Not sure what is happening here but obviously a reaction of some kind. Other chems are just Sulphur and Charcoal. Water and dextrin to bind.

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After about 20 minutes of searching I didn't find anything relating to the problem of tubes loosing length after pressing. Apologies if I missed it. i'm not sure what the correct term for this phenomenon is.

I am working on a 3/4" 1lb motor design after purchasing some tools from Caleb at Woody's. I live in the UK but just can not find decent tooling over here so spent a fortune on some quality stuff.

Anyways, my tubes are 7.5" long but lose close to 1/4" in length after pressing the whole thing using a 3 ton arbor. The nozzle presses in one increment, fuel grain in about 14 increments, delay takes about 5 and the bulkhead takes 2. I'm using a clam-shell support and when the finished motor is extracted it looks fine. No tube wall crimping or rippling and the things take off like lightening.

However, there is this shrinkage thing going on. What is causing this?

Tubes are convolute from Stell. Had 500 made up. They tell me they are made from the same materials their pyro clients use.

I am fairly new to pressing rockets. I have a PtoF gauge but I find just pulling by feel works well so far. It's a 3 tonnes so maybe I'm exerting way too much force without realising? 

Anybody else experience this sort of thing before?

I am interviewing my friend's step-mother Pauline on Friday in Huddersfield. She worked for Standard for about three decades and is the niece of Arthur Skargill. I want to write up an article for Fireworks and John B is very keen. Deadline for material is 1st May.

So.....does anybody on here have any suggestions regarding questions you yourselves would like to ask? Let's keep miner's unions to a minimum. I want fireworks stories, not Thatcher bashing anecdotes. Although I'll squeeze at least one of those in. How can I not?!

Pauline worked as an office clerk and she is baffled as to why I would want to interview her but already I have been told several interesting tales though not from the horse's mouth, yet.

I am unsure which decades she worked through but she is retired now so I am hoping 60's 70's 80's and possibly through the spiralling 90's when they gave in to China and Black Cat. Sad times for me those were.

Over to you.

Really struggling to find a supplier of silicon metal powder for fencepost prime. Any ideas?