zachy said:
brown color appears in the fire assay, when will weigh payon or turbinate, and it is not known to be? is also very fragile, and causes it to break, I use nitric acid at 22 and 32 baume and are washed with deionized water after each attack.
Zachy,
No offense intended but I'm having difficulties understanding you. For example, I looked up the word "turbinate" and, as far as I can tell, it means something like an inverted cone that is spiral-like??? For this reason, I'm going into the entire method I use. I'm sure that Westerngs and 4metals do things differently. Everybody does, at least somewhat.
What type of vessel are you parting in? Porcelain crucible? Test tube? Platinum crucible? Other? What is the silver to gold ratio in the bead before parting?
There are a lot of ways to part and a lot of different acid strengths used. Everyone seems to do it a little differently. I use Coors, high form, 30ml, porcelain crucibles for both parting and annealing. With these, there is no need to transfer the parted gold to another container for annealing.
http://www.sigmaaldrich.com/catalog/product/aldrich/z247138?lang=en®ion=US
I prefer for the gold to break up somewhat during the parting. That way, I feel more assured that the parting is complete. I use about a 4:1 to 5:1 ratio of silver to gold. In the white porcelain crucibles, you can easily see every gold particle and this will prevent losing any gold while pouring off the acids and rinses. I use a ratio of 7:1, distilled water to 70% reagent grade nitric, as the weak 1st acid. Your weak acid is stronger than what I use, about 2:1. I have found that by using a weaker 1st acid, the gold will break up less. For the stronger 2nd acid, I use the same strength that you do, a 2:3, water to acid solution.
Before pouring off any of the solutions or rinses, you may have a small gold particle or two floating on top. You can make them sink by touching them with the end of a glass stir rod.
About 45 years ago, I was very blessed to receive intense training in fire assaying by the main assayer, while spending a week, or so, 8 hours a day, in a large fire assay lab in San Francisco. The owner was a good friend of my boss. They had at least 6 or 8 big DFC, gas fired assay furnaces. It was a famous operation that had been there since the gold rush - the last one left, at that time. It's been gone for a long time. I can't remember the name.
Here is the exact same parting, rinsing, and annealing method that I was taught, in detail. I've never seen a reason to change it (at least, not much). It takes steady hands to do it.
(1) Flattening. Before flattening, I slightly squeeze the bead edge-wise using a pair of medium sized, clean, dedicated electrician's pliers. This loosens most of the bone ash, etc., stuck to the bottom. While still held by the pliers, I brush the loose stuff off. The tips of the jaws on the pliers are ground with a curvature that fits the curvature of the bowl of the cupel. I grab the bead in the cupel with the pliers and give it a slight twist to break it loose, then squeeze and brush the bead.
Next, I flatten the cleaned bead as much as possible with 4 or 5 hits using a clean, smooth, dedicated hammer on a clean, stainless steel anvil (a smoothed, polished 1"-2" piece cut from a 4"-6" dia. SS cylinder works well). I use a small, toothbrush shaped, SS wire brush gently to clean the bead, both before and after flattening. For the hammer, I use the flat end of a medium weight ball-peen hammer. When flattening, keep the hammer square and hit the entire bead. If you hit the bead at too much of an angle, it can squirt across the room and you'll probably never find it (don't ask me how I know this).
People that only use a 2:1 or 2.5:1 silver/gold ratio usually run the bead though a set of jeweler's rolls to get it quite thin and then form it into a cornet so it will fit into the parting vessel. Being thin, the acid will penetrate all the way through it. That is time consuming and, with a 4:1 or 5:1 ratio, it isn't necessary. Also, some people using the higher ratio just clean the bead and put it into the acid without flattening. I always preferred flattening. For one thing, it pulverizes any traces of bone ash left on the bead. It also parts a little faster because it is thinner.
(2) First acid (weak). Put the flattened bead in the crucible and cover with about 15ml of the weak acid. Heat on a medium-low to medium hotplate until no more reaction is seen (no more bubbles). Don't boil. Test for completeness by mashing the gold with the end of a small glass stir rod. You will see and feel any undissolved chunks of solid gold/silver.
(3) Pouring off. When complete, pick up the crucible with stainless steel lab tongs and swirl the solution at a slight angle until all the black gold particles are collected together at on side on the bottom of the crucible. If any of the particles float, they are submerged by simply pushing down on them with the end of a glass stir rod. Hold about a 3/16" glass stir rod across the top of the crucible and pour off the solution slowly and carefully down the stir rod, making sure no particles are lost.
(4) Rinsing. Rinse 3 or 4 times with hot distilled water and, after each rinse, pour off using the method in (3) above. In one place I worked we preheated the water in an elevated stoppered syphon flask with about an 1/8" ID Tygon tubing with a spring clamp, coming out of it. Add about 15ml of hot water to the crucible. In later years, I used a lab squirt bottle to add room temperature water to the crucible. In either case, while adding the water, rinse down the entire inside of the crucible. Put the crucible on the hotplate and, after it is steaming, heat for 3 or 4 minutes and pour off.
(5) Second acid (strong). Repeat with the stronger acid. The gold will usually lighten up somewhat, but will still be brown.
(6) Drying. After the final rinse, swirl until all the gold is collected together on one side, pour off the rinse water, and, without disturbing the placement of the pile of gold, put the crucible on a hotplate set at low temperature until the gold is bone dry. After drying, the gold color will still be brown. If you use too much heat, the gold will spit out of the crucible and you'll have to start over (here again, don't ask me how I know this).
(7) Annealing. Anneal the gold in the same crucible. For this, I carefully place the crucible on a triangle on a ring stand placed over a Bunsen burner, without disturbing the pile of gold. I tilt the crucible slightly so the side the gold is on is at the lowest point. Almost immediately, the inside of of the crucible will glow red and the gold will will change to a golden color. Place the crucible on a firebrick to completely cool. If placed on metal, the crucible will likely crack. If all is done perfectly, the gold sponge will usually be in one piece and will look very clean. If you see any white material at all on the gold or around the edges of the gold, it is dried silver nitrate and this indicates inadequate rinsing. If you see this white material either after drying or annealing, start over.
(8) Weighing. Dump out the gold into a tared disposable plastic weighing boat on the balance and weigh. If any gold particles are stuck to the bottom of the crucible (they are easily visible on the white background), they are easily dislodged with a needle mounted on the end of a small wooden dowel.
I have fiddled around with other techniques, crucibles, acid strengths, US Mint method etc., but always come back to this one. The above was written in detail and, therefore, it seems slower than it really is. I can do at least a dozen samples in an hour or less. It takes some practice, especially the swirling and the pouring off of the solutions and rinses.
Chris