Summary
- Measurements show that a used boar brush took around twelve days of hanging to get bone-dry, not just dry
- Lower humidity (water vapor in the air) was observed as causing faster drying of a natural-hair brush, even when the brush started with significantly more absorbed water in one test compared to another test
- Bone-dry mass of a natural-hair brush is not singular, but varies with humidity, since increasing or decreasing humidity causes hair to absorb more or less water, respectively. Mass versus humidity measurements confirm this behavior.
- Based on mass and temperature measurements, it appears that natural-hair brush mass is independent of air temperature
- It is advised that natural-hair brushes be dried for at least several days, preferably for at least two weeks, before long-term storage in closed environments. If brush mass is measured for a determination of bone-dryness, then using a scale with a resolution of 1 g would be inadequate, but a resolution of 0.1 g would be good and a resolution of 0.01 g would be even better.
Background
This experimentation was inspired by @1Cal's (Cal) recent mass measurements of drying brushes that were hanging and standing (link). In reply to Cal (link), I linked to a good summary on the brush-drying issue written by @147_Grain (Mike) in December 2015 (link). Mike wrote about how previous tests seem to show that the Simpson recommendation of drying the brush standing on its base dries the brush faster than the Kent recommendation of drying the brush hanging from a stand, but "that isn't necessarily conclusive" due to sample size, types of brushes, ambient humidity, and ambient temperature. I soon proposed my own test matrix of eight combinations involving my used boar brush vs. my new synthetic brush, "maximally wet" vs. "minimally wet" initial conditions, and hanging vs. standing drying methods while measuring mass, humidity, and temperature over time (link). The results presented here satisfy two of these eight scenarios with my used boar brush initially soaked heavily or lightly before excess water was flung/shaken out and the brush was hang-dried.
Experimental Setup and Measurement Process
The brush utilized here was a ten-month used Semogue 1470 boar brush with a 21 mm knot and a 50 mm loft. The brush was bone-dry before being soaked in room-temperature water. Water temperature was measured, as shown below.
The first test involved soaking the boar brush for about two hours, which is well beyond the typical few minutes of soaking time and seemingly well beyond the time need for full saturation (link). The water level crossed the handle in this test, such that the hair was fully soaked and the varnished beech wood handle was partially submerged. This situation is pictured below. In setting up the long soak, it was assured that there was effectively no trapped air bubble inside the brush.
The second test involved partially soaking the boar hair for a strictly timed two minutes. The water level crossed the hair, as shown in the picture below. As opposed to the first test, in which all of the boar hair became fully saturated with water, only some or most of the boar hair became partially saturated with water by the end of the two-minute soak of the second test.
For both tests, excess water was flung/shook out of the brush, the brush handle was then carefully dried of residual water, and then the brush was hung in the stand to dry next to a temperature and humidity monitor. Here is a picture of the setup:
The initial measurements of ambient humidity, ambient temperature, and brush mass were immediately made and recorded in an electronic spreadsheet for time being zero. Measuring humidity and temperature simply meant noting the readings from the temperature-humidity monitor. Measuring mass involved calibrating a scale with a 0.01 g resolution, picking up the brush from the stand, placing the brush on the scale, noting the equilibrated reading, and hanging the brush back on the stand. The scale is pictured below after having been calibrated with its calibration weight and after having the brush placed on the scale's platform. (The picture was actually taken when the brush was dry before the measurement process started.)
Humidity, temperature, and mass measurements were made over the course of thirty days with the first test preceding the second test. Testing was completed several weeks ago. For the first nine or ten hours of each test, measurements were made every half hour, and then generally after that, one or a few measurement sets were made every day during the testing period. 136 measurement sets were recorded for the first test and 80 measurement sets were recorded for the second test. The environment was kept fairly steady during this time. Indoor air conditioning was on some of the time, drafts were kept to a minimum, overhead lighting was kept mostly on, and the window, blinds, and curtains near the brush were kept shut the entire time. Further, through extra washing and drying of my hands, I made sure that my hands were clean before touching the scale, calibration weight, and brush. The scale was calibrated before each mass measurement, and the calibration weight was even cleaned on occasion with a clean cloth.
Results
Mass versus time is plotted below. (All plots here connect data points with straight lines.) The initial mass of the heavily and lightly soaked brush was 46.50 g and 43.71 g, respectively. As illustrated, the heavily soaked brush started with more mass, due to starting with more water, but it generally dried faster than the lightly soaked brush. Specifically, after the large majority of water had evaporated in each case after 1.3 days, the two-minute lightly soaked brush (blue line) had more mass than the two-hour heavily soaked brush (red line) by no more than about 0.15 g. This difference could be considered negligible and probably would not even be observed if the resolution of the scale were 0.1 g or larger.
The average air temperature was around 74 °F and 76 °F for the heavily and lightly soaked brush, respectively. The overall temperature increase from the first test with the two-hour soaked brush to the second test with the two-minute soaked brush was due to an increased outdoor temperature that was not fully countered by the indoor air conditioning. The plot shown below of temperature versus time illustrates how even though the temperature readings ranged from 70 °F to 79 °F and 72 °F to 86 °F for the first test (red line) and the second test (blue line), respectively, there was only a slight overall upward trend in temperature over the thirty-day testing periods.
Temperature may have played a role in how fast water was lost and the brush became bone-dry, but a correlation could not be established from the plot of mass versus temperature, as shown below. Temperature is shown as bouncing around, seemingly independent of brush mass, before and after the brush was considered bone-dry.
The reason why the two-hour heavily soaked brush dried faster than the two-minute lightly soaked brush, despite the fact that the heavily soaked brush started with significantly more water, is because the humidity (water vapor in the air) was significantly lower for the heavily soaked brush than it was for the lightly soaked brush before the brush became bone-dry. Humidity versus time is plotted below. The average humidity over the thirty-day testing period was 48 % for the first test and 51 % for the second test, which meant that faster drying was favored overall during the first test. However, faster drying was especially favored during the first test due to the significantly lower humidity before the brush reached the bone-dry state. The humidity difference when the brush was really drying is clearly visible, and so is the trend over time. Humidity significantly rose during the first test with the two-hour heavily soaked brush (red line), and then with the second test starting shortly after the first test finished, humidity basically picked up where it left off, but significantly dropped back down during the second test with the two-minute lightly soaked brush (blue line). This trend explains why, after the brush reached the bone-dry state, the mass tended to increase slightly with increasing humidity during the first test and the mass tended to decrease slightly with decreasing humidity during the second test.
The dependence of natural-hair brush mass on ambient humidity, and how there is no singular bone-dry mass for a natural-hair brush, is clearly shown below by the plot of mass versus humidity. It might be expected that mass would drop and approach a final, singular value, what one could call the bone-dry mass of the brush. It turns out, however, that there is no singular bone-dry mass for a natural-hair brush because of humidity. Increasing or decreasing humidity causes hair to absorb more or less water, which results in increasing or decreasing brush mass, respectively. In other words, natural-hair brush mass follows humidity as it goes up and down. This phenomenon may be difficult to observe when a natural-hair brush is drying, but as evidenced by the plot below of mass versus humidity, the dependence of mass on humidity is clearly visible when the brush is bone-dry. When the natural-hair brush is bone-dry, mass readings generally move back and forth along a positively sloped direction with humidity readings, creating a band of data points near the bottom of a mass-versus-humidity plot. This band of bone-dry data points will either express a linear or nonlinear (curved) relationship between mass and humidity.
Given variations in mass readings due to a scale's accuracy and precision, as well as variations in humidity readings, determining precisely when a natural-hair brush has reached the bone-dry state is somewhat subjective, though physically more meaningful than knowing when the brush has reached an arbitrarily defined "dry" state. For the two tests discussed here, it was found that the two-hour heavily soaked brush took about eleven (11) days to become bone-dry, while the two-minute lightly soaked brush took about thirteen (13) days to become bone-dry, taking longer than in the first test due to the higher humidity, as previously discussed. On average, the natural-hair brush used in the two tests thus took about twelve (12) days to become bone-dry. There didn't appear to be any water absorption issue with the varnished wooden handle, which wasn't even soaked in one of the tests, so reaching bone-dryness seemed to be limited to water loss from the brush hair only.
It should be noted that the 0.01 g resolution of the scale used here was critical towards making the conclusions of this study. As illustrated above, the bone-dry mass of the natural-hair brush tested increases by approximately 0.2 g as the humidity increases from 40 % to 60 %. If a scale with a 1 g resolution were used, then the resolution would have been much larger than the bone-dry mass difference over the tested humidity range and the relationship between bone-dry mass and humidity would not have been found. Further, given that it took less than one day of drying for the mass to be within 1 g of the bone-dry mass zone, it would certainly have been concluded that the brush became bone-dry in much less time than it really took. Using a scale with a resolution of 0.1 g would have been good, resulting in a fairly accurate determination of the time to reach bone-dryness, but the dependence of mass on humidity might have been missed. If there is a slow, long-term drying of my used boar brush's hair that was not captured over the thirty-day test period, then a longer test period would have to be employed to observe the behavior, especially if using a scale with a resolution larger than 0.01 g.
Advice
It isn't necessary by any means to measure mass in order to determine whether a brush is dry or not. The senses of sight and touch are usually enough to figure it out. However, using the senses to distinguish between dryness and bone-dryness can be difficult or seemingly impossible. To be on the safe side, it is advised that natural-hair brushes be dried for at least several days, preferably for at least two weeks, before long-term storage in closed environments in order to minimize the chances of bad microbial growth. If it is desired to measure mass in order to determine bone-dryness, then one must use a scale with a resolution of 0.1 g or better. No matter the resolution, the capacity of the scale must be large enough to accommodate the brush, of course.
Future Work
I would like to continue with bone-dry brush experiments, specifically following my proposed test matrix discussed in the background section above. My next tests would be with a hanging synthetic brush. It is possible that there might be some dependence of mass on humidity for a synthetic brush, but without natural hair to take on and lose water, I expect that a synthetic brush's mass would be totally or fairly independent of humidity, resulting in a singular bone-dry mass. Different brushes could be tested regarding hair type, knot diameter, and loft height, and there could be value in that, but the complexity is daunting. The fundamentals are more interesting to me. The test matrix that I proposed was focused on determining which drying method, hanging or standing, dried brushes the fastest. That issue remains open, but at least the research presented here helps us understand the importance of humidity on the drying rate, how there might not be a singular bone-dry mass, and how long it can take for a brush to become bone-dry. Future experiments must involve careful attention to humidity.
