What Causes Casting Marks?
7 Minute Read
In the Spring 2003 Issue we asked the question "What causes these marks on this casting?" Around 100 readers responded to the question.
Everyone that responded said the same thing; Water Marks. Investment needs to be finished and set aside in 8 to 8 1/2 minutes - not 5 or 6 minutes. Water marks are caused by finishing too early and allowing the investment to sit to long and settle before it's ready to begin hardening.
We received the most in-depth, comprehensive paper on the subject from Eddie Bell at Rio Grande, which we wanted to pass along to all of our readers. I suggest that every bench jeweler read this very informative letter. Although we would like to publish each of the responses, we do not have the room to do so. We do appreciate your responses and Thank You for taking the time to write to us.
- Chuck Koehler
I'll take a shot at the question you pose in your interesting article in the Spring 03 issue of Bench Magazine.
It is always dangerous to diagnose a casting defect without knowing all the particulars of the process used and examination of the casting; but what fun is it to play it safe. From the picture the defect looks like something we call Water Marks or Water Trails. Water Marks are raised additions to the casting that are found on vertical surfaces of casting or on the underside surfaces that were between vertical and horizontal when the investment was setting. They are the product of investment erosion at the interface between the wax pattern and the investment caused by the water and powder separating.
Water Marks are considered a process problem, in other words, punishment for not following instructions carefully. The most common cause is not mixing for the prescribed time. Most investment manufacturer instructs us to mix the investment for 5 minutes then vacuuming in the mixing bowl for 1.5 minutes, pouring and vacuum in the flask for another L5 minutes. The total working time is g to 9 minutes as you say. I can see that this casting didn't take very much investment, and I know how convenient it is to just whip it up and get it done.
We can get a small amount like that mixed smooth as cream in just a couple of minutes. So why mess around mixing another 3 minutes. What a boring waste of time. When the engineers were designing that fine product for all us casters, they were thinking you could afford three minutes mixing time better than the time required to grind off the water marks, not to mention the metal that could have been recycled if it was in the sprue instead of in the dust collector. The investment binder (plaster of Paris) needs time to go into solution with the water and then start gelling, and it simply doesn't happen faster than nature will let it. If you have 4 minutes left on the timer set for 9 minutes you didn't mix long enough. You are correct that you can't exceed the working time, but you need to use all of it every time.
Somebody will say, ah ha, I did mix it for five minutes and I still have these defects; what do you say about that? Well, I could start by saying that there are a few other bits in the instructions like the powder to water ratio you mentioned. I really do hope that you weigh the powder and measure the water carefully, because too much water will also cause water marks and make the investment weak to boot. Slurry temperature is another possibility, but not too likely in this case, but if the water and or powder is too cold, it will extend the working time and therefore the mixing time. Inversely, if the slurry is too hot the working time will be shortened. Ifthe investment powder is at room temperature and the water about 80'F you should get consistent results.
What's that you say? You did all the above just right and you still have these marks / (You didn't think it was so complicated did you?) Well, a couple of years ago Ralph Carter gave a paper at the Santa Fe Symposium on Jewelry Manufacturing Technology that may explain the last possibility and that is hard water. Yes, for years we would see water marks from time to time with no explanation and then his good research tums up the facts. The minerals in tap water can greatly extend the working time of investment, so if you have hard water we now recommend that you use distilled water to mix investment.
A measure you didn't mention is the gloss off time. That is when the investment sets and then the watery gloss goesaway. You can test your investment and your water by mixing a small amount with the proper ratio of water in a paper cup. I just mix it with my finger for 5 minutes and time from the moment the water and powder are introduced to the time the glass is lost. If it is longer than 12 minutes, try it again with distilled water. If the gloss off time is 12 minutes with distilled water, then you know your water is too hard and you should use distilled water instead. Oh! I forgot; here is one more. Sometimes old investment will have an extended gloss offtime. If you test it with distilled water and the gloss off time is longer than 12 minutes, I guess that could cause water marks too, (I never tested this as a water mark cause but it makes sense to me), but don't panic, just add one minute to the mixing time for every minute to gloss off over 12. For example if the gloss offis 14 minutes instead of 12, mix the investment for 7 minutes instead of 5.
A couple of other hints while I have you here, keep the mixing bowl, mixer, flask and anything else the investment will come in contact with very clean. Previously set investment is a powerful accelerator to the working time of investment. Mixing investment in a dirty bowl can greatly shorten the setting time and some bad things can happen, like bubbles on the castings. If you wash the mixing tools before the investment sets, it is easy to keep them clean.
Pounding on the vacuum table is good exercise, and I did it for years, but if the vacuum pump is good, controlled test show that it isn't necessary. That said, it doesn't hinder anything ether. One reason to have the slurry temperature just warm is that water boils at room temperature in a vacuum. It is the boiling action that helps carry the air to the surface and out of the investment.
Like any boiling water, it makes water vapor that gets carried into the vacuum pump. That is why draining the water out of the vacuum pump should be done religiously the next morning after investing, don't leave it in there for a week to rust the pump parts, and don't run the pump with water in the oil. If the oil turns frothy and a dark cream color you have oil-water meringue. Turning water into vapor consumes a lot of energy and that energy is taken out of the slurry with the vapor until the water gets too cold to boil. That is when the boiling slows and then stops. If you put a thermometer in the investment while it is in the vacuum, you will see that the temperature will go down because unlike boiling water on the stove, the lost heat is not being replaced by a burner.
In commercial casting operations we don't like to top offthe flask. I know you can get away with it, but more consistent results arrive from using a flask extender and pouring the flask full before vacuuming and then pull off the flask extender as soon as the flasks are removed from the vacuum. The flask extenders are easy to wash before the investment sets. Why? The poured investment slurry has a velocity that will carry it to the interior of the flask. Air can be entrained and deposit bubbles on the pattern.
I can't leave without putting in a plug for safety. Investment is about 25% Plaster of Paris and the rest is silica. Very small particles of silica, too fine to see with the naked eye, can be taken into your lungs and once there can cause a deadly disease called silicoses. You are most exposed when you are weighing the powder and when you quench the flask. Wear an approved respirator and when quenching, get the whole flask under water fast and keep it there until all the investment is cool. Letting the super heated investment break the surface puts plums of silica containing steam right in your face.
Good protection from investment dust is necessary wash investing areas with water, and don't do anything that gets the dust into the air. My doctor tells me that smokers are 100 times more susceptible to silicoses than non-smokers. We send everyone here that works with investment for a pulmonary function test every year so they can see if lung capacity starts to deteriorate, and smokers are not allowed in the casting area.
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