Re: Lithium palladium vs. normal palladium
Please let me begin by indicating that the maximum black density possible from the materials should not be utilized in the print. The actual maximum black in a print (and this is likely so for any process) is less dense than the capability of the materials. This can be demonstrated printing from a calibrated step wedge. If the darkest tone is printed to maximum possible density, visual discrimination will be lost between the other dark steps. Attempting to correct this in the negative will likely lead to overexposure, hindering the highlight values. Just as film ‘works’ within a certain range, so too the print materials have their range. Note: The quest for maximum density and superfluous densitometer readings only serves to hinder one’s ability to ‘see’ the print. From experience with making prints, there seemed to be a threshold of the amount of noble metal per area in the print which produces the ‘richest’ print. At this point more metal does not contribute to the look and less results in a weaker look. A study was made to determine this threshold by the evaluation of several series of prints made with various solution strengths of sensitizer and double metal salts. The bulk of this work was done using ferric oxalate sensitizer (FO). Details of this and other studies are presented in my guide so others wishing to do so can make their own evaluations. The calculators in my guide were based on this study. Either reducing the metal (by using less solution or a more dilute solution of the double salt) or reducing the sensitizer (either the amount or solution strength) from the threshold produced weaker prints. The resulting prints corroborated the metal to sensitizer relationship that is used in the calculators. Additional series of prints were made using ammonium ferric oxalate (AFO) starting with the assumption that the metal relationship might be the same as with FO. A threshold was found using AFO consistent with that for FO. However, the solution strength of the AFO needed to be greater than that for the FO. A 40% solution of AFO worked well with no benefit of increasing to a 60% solution. But also, the amount of metal needed was found to be more than that needed when using FO. This and other studies demonstrated that there must be a certain coverage (metal per area) which was found to depend on coating efficiency and the purity of the sensitizer (assuming using pure metal salts). The coating efficiency can vary with substrate, the coating tool and technique. I have found powder FO purity to vary from 85% to 98%, however all working the same once adjusted during mixing. For small prints, coating solution sticking to the mixing vessel can significantly alter the efficiency. In summary, a ‘rich’ print requires consideration of all the following: Maximum black in the print is less dense than maximum density the materials can produce. There exists a threshold of metal per area to produce the ‘best’ maximum black. The purity of the FO will adjust its weighing for the sensitizer solution. The coating coverage can be influenced by the substrate and it is important to keep the coating within the specified area. The coating efficiency needs to be determined for each substrate, coating tool and technique. Note: If one does not mind wasting some of the expensive materials, slightly more and slightly stronger solutions can be used which could save the time and expense of reprinting a borderline threshold print. Also as Witho pointed out, one may set their maximum black at a lighter value which might alter the optimum threshold and subsequent factors for that scenario. Pleasant printing, -- Jeffrey D. Mathias http://home.att.net/~jeffrey.d.mathias/
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