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  1. Iron darkens rosin and commercial makers of rosin go to great lengths to prevent iron contact (SS reactors, metal chelating agents) in order to make the rosin as light as possible. Light colored rosin sells at a premium. Heating rosin in a cast iron pot is very sensible if your goal is to make it dark. I noticed in many previous threads discussion about using hot plates, various means of temperature control, and the problem of uniform heating. In my profession, hot melt adhesive chemist, we would never melt resins on a hot plate or stove. Ovens are much safer, easier to control, and heat uniformly. Most importantly, remember that rosin vapors are harmful, and hot rosin is a fire hazard.
  2. We do a lot of thermal stability testing of rosin and rosin ester containing materials by aging a few hundred grams at 180 degC for up to two weeks. We monitor color and viscosity change and record skin/char formation etc. We use both unlined pint paint cans and glass beakers and over the years have not noticed the container having any effect on thermal aging. With regard to iron, it's one of the main factors that affect color during rosin production. There are some patents on various techniques to reduce iron content to yield light colored rosin. If you are heating rosin to darken it, I'd think some iron shavings would be beneficial.
  3. This is an old thread that surfaced after I searched for "shrill e string". I'm trying out a violin that has a perfect sounding G, D, and A for my tastes, but the F# and G of the e string could "break a goblet" as a previous poster said. The e string was not like this in the shop, where the humidity was controlled, but I've had the violin in my humid basement for the last three days. Could it be, as some of the previous posts imply, that the humidity has caused the sound post to extend and become too tight? That that is the reason for the shrillness?
  4. Not true. Professionals are sued all the time for giving advice and consult that someone can prove caused harm. That's why they sell professional liability insurance.
  5. I've been an industrial chemist for 41 years and I work with resins like this all the time. I've never made a varnish, but blending, mixing, reacting, and testing things like this are what I do. On the side, I've been characterizing bow rosins and have just finished measuring the Tg's of 20 commercial ones. I suspect, and some literature supports, that Tg is the single largest measurable characteristic that would correlate with "playability", whatever that is.
  6. These are glassy, amorphous materials which strictly speaking don't have a "melt point". Instead they have a "glass transition temperature" (Tg) which is analogous. Melt points usually refer to crystalline materials. While melt points usually happen over a very short temperature range, the Tg happens over a wide temperature range. A typical Tg for rosin ranges from 50 to 70 degC with the peak around 60 degC. Hardness is a vague term, but in this context it's often measured with some kind of needle probe such as a penetrometer or Shore hardness gauge. At a temperature far enough below the Tg, the hardness isn't affected too much by the Tg, but as the temperature increases the material with the higher Tg will act harder longer. Liming the rosin raises the Tg so it would appear harder at higher temperatures than unlimed rosin. I'm curious, do people make varnish from rosin esters? There are glycerol and penterythritol esters of rosin that raise the Tg and neutralize the rosin as much as liming does. Esterification also stabilizes the rosin for improved oxidative resistance.
  7. The amount of lime you use to neutralize the rosin acid would depend on the purity of the lime and acid number of the rosin. Knowing these, you can calculate the correct amount of lime to use. Otherwise, it's a matter of trial and error. The amounts in the video may not apply to your materials.
  8. Could you be using too much lime? Once the lime neutralizes the rosin acid, I assume the excess acts like a filler and would make the mixture opaque.
  9. Here's a link to an informative 1940 patent on making limed rosin: http://www.google.com/patents/US2237973
  10. The Naval Stores Rosin Color Scale is independent of the kind of rosin. Tall oil, gum, and wood rosin are all graded according to the same scale. Something like "WW" denotes the color. Nothing else. see http://www.ssco.com.tw/Tintometer/3000series_comparator/AF607ROSIN.pdf It's more common these days to specify the color of rosin and similar resins using the Gardner Color Scale. see http://www.gardco.com/pages/color/liquidcolor_stds.cfm
  11. As an industrial chemist who has worked with rosin and related materials for a lifetime, I think some of you are being too cavalier about the hazards of cooking over a flame. The fire and burn hazards are obvious, but the health risks of exposure to the vapors should not be ignored. Rosin is a strong allergen and though you may not react to it now, continued exposure can sensitize you (and your neighbors). People have strong and sometimes life threatening reactions to rosin vapors. In the US, OSHA has set monitoring and exposure standards for the electronics industry where rosin is used as a solder flux.
  12. If all you want to do is darken the rosin, why not add some metal (iron, copper) shavings to the heated batch? It should dramatically increase the rate of darkening. Manufacturers of rosin go to great lengths to eliminate the presence of iron because their goal is to make it as light as possible. There are some old patents on this. ~0.01% carbon black will do it too.
  13. Putting your dismissive and insulting tone aside, I fail to see your point on bow/string slip-stick. Whether a peg "pops" or not is a matter of static friction. Bow stick-slip is much more complex and includes a combination of dry friction and viscoelasticty and the effect of shear rate, temperature. pressure, and who knows what else. I've done enough research on stick-slip to know that no one knows what's going on at a fundamental level. You are correct that rosin and wax have different material properties. Rosin is amorphous and wax is crystalline. But they share the common ability to form high friction films which is the property needed for a peg compound.
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