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Wet Dry Drying Study
The purpose of this study was to investigate any differences that may have existed between exposing a coating that has been dried “Wet” or “Dry”.
Drying "Wet" means that the relative humidity (RH) of the coating was at ambient and relatively high (typically 60-70% RH) when exposed. Drying is accomplished by blowing ambient air over the coating until just dry.
Drying "Dry" means that the ambient RH was relatively low (preferably less than 40% RH) and the print dried by blowing air with medium heat over the coating to dry as much as possible. This method is that which is also referred to as drying to “bone dry”.
Note: A method to check if the RH of a paper has reached equilibrium with the ambient, is to place the paper on a scale accurate to 0.01 grams and observe the movement. An increase will indicate an increasing RH in the paper; while a decrease will indicate a decreasing RH in the paper.
This study was made after preliminary work from other studies. Preliminary study demonstrated some interesting results.All weights were measured with an Ohaus Cent-O-Gram mechanical quad balance scale accurate to 0.01 grams.
- It became apparent that the paper may change water content rather quickly.
- Solarization effects may occur - which are addressed later.
- A printout which was unusually very dark and of extremely high contrast was only seen with the “wet” dried coatings at an ambient temperature of 82oF. However, this high of a temperature produced its own problem as discussed in Solarization later.
- Pre-humidification of the paper had to be re-thought and is also discussed in Solarization.
- As a result of preliminary study, the “Dry” portion needs to be repeated.
Ambient Conditions
For the “Wet” part, the temperature was 72oF and the relative humidity 62%RH for A, B, C, and D; and the temperature was 75oF and the relative humidity 68%RH for E.
For the initial “Dry” part, the temperature was 77oF and the relative humidity 45%RH for A, B, C, and D. Plans are to repeat the “Dry” portion once the RH can be kept below 40%.
Materials and Procedure
- Paper was Crane's lot # 5302 (AKA parchment Business card stock, Cover 90, “platinotype”). Paper was cut to slightly larger than area to be coated.
- This paper was chosen as it is a thick paper having a capacity to hold moisture.
- Coating was 9 drops FO (27% solution), 6 drops K2PdCl4(19% solution), 3 drops K2PtCl4 (24% solution), 1 drop Ammonium Dichromate (0.25% solution).
- This coating mixture is typically used.
- Coating mixture was weighed.
- Paper was weighed.
- Paper was humidified only briefly on the coating side with a sonic mister. (See discussion of Solarization later.)
- Paper was weighed and verified that there was no increase in weight. (See discussion of Solarization later.)
- Coatings were applied by brush and covered enough area for a 4x5, and a 21-step.
- Coated paper was weighed.
- Coatings were dried by one of the methods above.
- Dried coated paper was weighed.
- Coating exposed for 10 minutes with BL type lamps through a negative and 21-step.
- Exposed paper weighed.
- Developed in Potassium Oxalate (1 minute)
- Rinse/Pre-clear in water bath (2 minutes)
- Cleared with phosphoric acid baths for a total of 30 minutes
- Rinsed in buffered water, washed, dried
- Dried, finished print weighed.
Weighing Data
“Wet” - weight in grams A B C D E mixture weight 1.25 1.25 1.25 1.25 1.25 pre-misting 4.91 4.76 4.46 4.15 6.08 pre-coating 4.91 4.76 4.46 4.15 7.01 post-coating 5.93 5.82 5.47 5.12 7.08 post-drying 5.25 5.02 4.72 4.44 6.34 post-exposure 5.01 4.68 4.39 final 5.00 4.86 4.55 4.28 6.04
“Dry” - weight in grams - Data questionable due to Pre-misting A B C D mixture weight 1.25 1.25 1.25 1.25 pre-misting 9.26 8.73 8.71 8.71 pre-coating 9.39 9.12 9.08 9.39 post-coating 10.24 10.00 9.92 10.29 post-drying 9.04 8.75 8.72 8.92 post-exposure 9.10 8.75 8.76 8.98 final "Dry" data will be obtained in which the pre-misting is omitted or reduced so that no gain in weight is detected, once the RH can be kept below 40%.
Calculations
“Wet” - changes of weight in grams A B C D E Due to misting 0.00 0.00 0.00 0.00 0.03 Due to coating 1.02 1.06 1.01 0.97 1.01 Start to Dry 0.34 0.26 0.26 0.29 0.26 Due to exposure -0.01 -0.04 -0.05 Start to final 0.09 0.10 0.09 0.13
“Dry” - changes of weight in grams - Data questionable due to Pre-misting A B C D Due to misting 0.13 0.39 0.37 0.50 Due to coating 0.85 0.88 0.84 0.90 Start to Dry -.22 0.02 0.01 0.03 Due to exposure 0.06 0.05 0.04 0.06 Start to final The weight of the coating chemistry minus water except for the 6H2O attached to the FO is calculated to be about 0.0203 grams assuming none lost.
Observations
SOLARIZATION
"Wet" prints A and B showed no solarization.
"Wet" print D barely showed a minute solarization.
"Wet" print C showed very little solarization.
"Wet" print E showed some solarization. The RH of E was intentionally forced upwards; note also
the increase in weight due to misting.
"Dry" prints that received a strong pre-coating misting showed solarization.
"Dry" prints will be repeated without a weight gain from pre-coating misting.
In every example the solarization did not show while the prints were wet from processing; only after drying did the solarization become apparent. This hiding of the solarization is suspected to be related to the depth that the coating has penetrated the paper; when wet, greater depth is viewed into the paper. (This is also why prints appear to have greater depth and substance when wet.)Whether what is happening is actually solarization is not known. What had been observed, and now related to certain conditions, appears similar to a solarization. What is important is that this effect can be avoided or significantly reduced by controlling a couple of the parameters. It is expected that different papers will behave differently, so the conditions set forward in this study only apply to the paper used.
Previous experimentation demonstrated that the solarization effect was only observed at higher ambient temperatures (82oF) and when the pre-coating misting was overabundant. When the ambient temperature was kept lower than 75oF and care was given so as to only just barely mist the paper prior to coating, no or very little solarization was observed. All other parameters had been kept constant.
It has been known for some time that papers generally coat better when cold, and that more mixture is required to coat the same area at a higher temperature. It is suspected that the higher temperature encourages the chemistry to soak deeper into the paper. This is paper dependent; and is much more noticeable in a thicker paper which absorbs moisture.
It has been thought that pre-coating humidification of most papers was beneficial as it made the paper more susceptible to accepting the coating. However, this must now be re-thought, as too much humidification (misting) may now promote the solarization effect. It is suspected that a rise in relative humidity above the ambient will encourage the coating chemistry to soak too deeply into the paper. At a temperature of 73oF, an over misted paper led to solarization, whereas no misting provided an excellent print with no signs of solarization.The solarization effect is suspected to be caused by the deep absorption of coating chemistry into the paper. It is desirable to get the chemistry into the paper, but it appears now to a certain limit. This limit can be controlled by a joint effort of command over the temperature and the pre-coating misting. Each paper will have its own characteristics. For the Crane paper used in this study, it is suggested that the ambient temperature be kept below 75oF. For any paper it is suggested that the misting be quick enough to not increase the weight of the paper (measured to at least 0.01 grams).
PRINTOUT
Another effect of the higher temperature was a dramatically increased printout. This printout was extremely high contrast and dark. It is thought the this printout would produce a better separation of shadow detail. This increased shadow detail was observed in some prints, however these prints also exhibited a large amount of the solarization effect. It would now become a balance of shadow detail for solarization. It does seem that the solarization could be controlled to the very darkest areas while the increased shadow separation could result in an overall benefit to the print. This was not attempted for this study; the temperature was kept in the low to mid seventies and all tests kept misting to a minimum so as to not cause an increase in weight. This printout may have merit and should be studied further.
PRINT COMPARISONS
To be completed after obtaining "Dry" data.
It is suspected that prints will be almost identical, but more data must be collected to verify this.
And, this could be paper dependent.
WEIGHT COMPARISONS
To be completed after obtaining "Dry" data.
A weight of interest here is the start-to-finish which is expected to give an indication of the difference in water content between the "Wet" and "Dry" methods. If it is assumed the the "Dry" method produces no weight increase, the amount of water in the "Wet" method, for the given ambient conditions, would average 0.28 grams. Plans are to obtain this weight for several ambient RH, and determine the temperature sensitivity, IF... If there is a notable change observed in the print. Thus far it is suspected that prints will be almost identical.
CHEMISTRY CONSUMPTION
Considering "Wet" prints only.
Note that not all of the coating chemistry was delivered to the paper as some was soaked up by the brush and some stuck to the shot glass. If it is assumed that the chemistry delivered to the paper is (weight of post-coat)-(weight of pre-coat) or delta due to coat, this would indicated that a bit more than 81% of the chemistry was delivered to the paper.
Conclusions
Temperature and pre-coat misting can be controlled so that the solarization effect can be avoided or minimized.
To be completed after obtaining "Dry" data.
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