With our sample set chosen, our substrate selected, and our sample mounts determined, we next needed to devise a process for applying the dyes. Since the ultimate treatment would likely involve airbrushing, we chose to spray deposit the dyes onto the quartz plates. For this we had to consider and anticipate the different requirements and challenges that might arise in the application, including:
- What is the best technique for mixing the dye solutions?
- What delivery system gives us even coverage and the most control?
- How much dye should be applied and how can we measure that?
- What shape should the dye film deposit be to allow repeatable color measurements?
- How should we mask the substrate to achieve the desired sample shape?
- How do we economize and reuse the substrates?
1. Mixing: Our mixing procedure reflects the method reported by Ciba-Geigy (the manufacturer of Orasol® dyes before its acquisition by BASF in 2008) in the literature describing their product testing. [Ciba_orasol_brochure] Where Ciba’s protocol is unclear, we have standardized our method based on our experience of what works well.
The powdered dye is measured and then incorporated into a volume of solvent to create a 1% (weight to volume) solution. The solution is stirred using a magnetic stirrer for approximately one hour. After mixing, the solution rests for approximately 24 hours. During this period, undissolved dye particles settle, and afterwards the solution can be decanted into a new glass bottle.
We have observed that when left in storage, the dye solutions appear to have a shelf life. In some cases we have seen changes in the color of the solution and the formation of crystalline deposits. We haven’t looked into what these changes might mean for the behavior of the solution. We have just taken them as indications that something has changed, and in response, we prepare fresh dye solutions for each round of sample production.
2. Delivery System: We elected to apply the dye to the quartz plates using an airbrush. Airbrushing is, in general, the delivery method that permits the greatest range of expressive use of the dyes in taxidermy restoration, where controlling color value, gradation, and blending is critical to achieving a successful result. For the same reasons, it is easier to control the amount of dye deposited onto the plate using an airbrush than by brush, roller, dropper, or dipping techniques. The spray application facilitates putting down a thin, even layer, which can be gradually built up to the desired level.
Our dye solutions are applied to quartz plates using an Iwata Eclipse HP-BCS airbrush at 20 psi of air pressure from a compressor. Of course, having elected to use the airbrush, our samples are always prepared in a fume hood while the researcher is wearing appropriate personal protective equipment.
3. Controlling Dye Application: It is important to control the dye application because the color produced by a medium-value application is inherently more light-sensitive than a heavy or light application of the same color. The sensitivity of the medium-value color derives from its reflectance spectrum having values that are not pinned at the low values (for very dark colors) or high values (for very light). It has also been found that for a given amount of dye loss, reflectance values around 30-40% show the largest increase. So when we spray our dye solutions on the quartz plates, we are attempting to produce samples whose reflectance spectra have minima that fall in that range. Doing so also avoids pinning large parts of the spectrum at the very high or low reflectance levels. (Specifics of reflectance spectra will be described in an upcoming post.)
However, controlling the amount of dye deposited onto the plate, even in approximate terms, presents significant challenges. Theoretically it would be possible to accurately measure the weight of the dye deposited, but adjusting one’s delivery to repeatedly match this measurement is fanciful. A more realistic approach to dealing with this uncertainty is to aim for the most sensitive application in which color change can be accurately measured and described; that is, the lightest application that is not so light as to make impossible the observation and measurement of fading.
Experiments conducted for the paint industry have demonstrated that such samples have a minimum reflectance of 30-40%; that is, in the spectrum of light reflected from the sample surface, the amount of light reflected at the most readily absorbed wavelength is 30-40%. When this condition is met, the entire spectrum will fall between the extremes of light and dark, so that there is room for it to gradually rise or fall as the colorant fades or darkens.
Since it can be difficult to visually gauge whether a particular dye sample on a clear plate meets these criteria, we created a set of small mockups with white backgrounds that reflect a range of minimum reflectance values (14% – 76%). These give us a sense of what each of those dye loadings look like. To arrive more precisely at the 30-40% minimum reflectance value in our samples, in the course of spraying, each sample is placed on a white membrane filter backing and measured using the spectrophotometer. More dye is added, if needed, until the desired loading is achieved.
4. Sample Shape: As our last post mentioned, our experimental procedure calls for taking periodic color measurements from each replicate throughout the duration of the exposure. At each time index, the values obtained from three locations on the plate are measured and averaged, to produce an overall measurement for the sample. In order to accurately calculate color change (which we will discuss in depth in an upcoming post), it is important that these three locations are reproducible, i.e., they do not change from measurement to measurement.
This requirement informed the design of a small jig with a wide T-shaped window into which the quartz plate can be inserted in any of three positions: left, right, and bottom. The spectrophotometer is placed in a fixed position atop the window. As each set of measurements is taken, the plate is moved through the three arms of the T. As long as the plate is inserted into the jig in the same orientation each time, the three measurement locations are constant. To ensure proper orientation of the quartz plates for each round of measurement, the dyes are applied to form an inverted T-shaped deposit on the plate. This irregular shape ensures the plate is properly orientated to provide the three viable measurement locations. If the plate were positioned in the jig in an incorrect orientation, at least one measurement would include an undyed area on the plate, giving the researcher an immediate cue that she has made a positioning error.
5. Masking: A metal mask easily clips on to the plate to expose the same T-shape sample area in each application. After application, the metal mask is removed and cleaned before applying the next dye solution.
6. Reusing substrates: Since our quartz plates are in limited supply, their use does not end after a test cycle. After final color measurements and final photography, the plates are cleaned so that they can be reused for the next test round. Our cleaning procedure is intended to ensure that the plates are free of contaminants prior to the application of dye. Although our substrate is chemically inert, contaminants or dye degradation products can interfere with the results. Through testing and troubleshooting, a streamlined process is now in place:
Quartz Plate Cleaning Procedure:
- The quartz plates are first wiped with a solvent to remove grease and/or remaining dyes left on the surface from the last completed test cycle. The solvent we have chosen to use for this process is acetone, as it is effective on grease and all of the dyes.
- Solvent cleaning alone is not sufficient to fully remove the dye and all residues from the plates. The initial solvent cleaning step can redeposit dye in the roughened edges of the plate, where it becomes stubbornly embedded. Furthermore, we found that many dyes as they age create a thin, transparent, insoluble film on the plate – a “ghost” pattern on the surface, which accepts new dye solution differently than the virgin quartz surface. To remove these residual deposits and “ghosts,” we next polish the plates with Bueller MetaDi Monocrystalline Diamond Polish, 1 μm. The monocrystalline diamond particles are single grain particles with sharp edges. These particles are suspended in a fluid, which we can spray onto the plates and gently buff the surface of the quartz using cotton pads. The polish removes the residual dye and insoluble deposits on the plates.
- Next, the quartz plates are transferred to a bath of detergent and water. The detergent, Sparkleen, (Sodium Carbonate 10 to 25%, Sodium Dodecylbenzenesulfonate 1 to 10%, non-ionic detergent 1 to 10%) is a conventional laboratory glassware detergent that aids in the removal of both organic and inorganic deposits.
- Finally, after the quartz plates have been rinsed of the detergent, they are allowed to dry and then receive a final wipe with acetone. This final round of solvent cleaning is intended to ensure that no contaminants are left on the plates from handling. Once the plates are clean, they are ready to be sprayed with dye once again.
The preparation of the quartz plates is both time-consuming and stringent, but is an essential part of the experimental procedure.