Jps pt/Pd paper

4 SENSITOMETRIC RESULTS

All the sensitometric tests were carried out by exposing the coated paper in contact with a 0.15 logH increment step tablet to the ultraviolet light source described in §3.5, with the contact printing frame distant 8 cm from the lamps. The papers were dish-processed with manual agitation according to the procedure in §3.7.

Diffuse reflectance densities were measured and the characteristic curves (D/logH) of diffuse reflectance density versus log(relative exposure) are plotted in Figs.2, 3 and 4 for platinum, palladium and mixed platinum-palladium sensitizers, dried to the relative humidities specified.

Table 4. Summary of Sensitizer Parameters

Sensitizer	R.H.%	Relative Speed	G	logH	Colour
Platinum	55	1.66	1.46	1.2	warm black
	80	1.00	0.96	1.5	neutral
	91	0.49	0.81	1.8	neutral
	100	0.41	0.65	1.9	grey-black
Palladium	32	0.51	0.41	2	red-brown
	55	1.29	0.66	1.9	purplish-brown
	80	2.51	0.74	1.8	neutral black
	100	2.24	0.66	1.9	neutral black
Pt/ Pd (3:1)	20	1.23	0.97	1.6	warm black
	55	1.02	0.7	2	neutral
	80	1.07	0.7	2	neutral
Pt/ Hg (10:1)	0	1.70	1.15	1.2	sepia
	55	1.70	1.02	1.5	warm black
	80	1.70	0.90	1.65	warm black

Table 4 summarises the values of the contrast both as maximum slope, G, and the approximate log exposure range, logH, between density values of fog + 0.04 and 0.9Dmax; the speed is assessed on the basis of exposure needed to produce a print density of 0.75, and is expressed on a relative linear scale referred to a platinum sensitizer at 80% R.H. as 1.00. The colours of the images are also indicated.

4.1 Contrast

It will be noted that the characteristic curves for platinum resemble those of modern silver-gelatine printing papers, but the platinum sensitizer has a lower contrast, corresponding to a 'Grade 0' paper. The D/logH curves for palladium, however, display an unusually long toe region, which can confer great delicacy on the upper print tones, and the overall contrast is even lower.

It is clear from Fig. 2 that both the contrast and speed of the platinum sensitizer decrease with increasing relative humidity, while Fig. 3 shows that the converse is substantially true of the palladium sensitizer; both systems converge on a common value of G = 0.66 at 100% relative humidity. The behaviour of a mixed sensitizer, having the molar ratio Pt:Pd = 3:1, as shown in Fig. 4 is therefore understandable: between 55% and 80% R.H. the curve is almost identical with a G = 0.7, showing little variation with relative humidity; only by drying to 20% R.H. is a significantly higher contrast obtained.

4.2 Speed

As judged by the exposure required to produce mid-grey (D = 0.75), the palladium sensitizer is two and a half times faster than platinum at 80% R.H.. The speed of the mixed platinum-palladium system is comparable with that of the pure platinum system, and varies little with relative humidity. Its constancy in this respect recommends its use when the control of relative humidity is difficult.

4.3 Maximum Density

The maximum optical densities obtainable with the sensitizers were not reached in all tests, in order not to prolong exposures unduly which can lead to heating and desiccation of the samples. However, the values of Dmax were found in a number of cases to be typically in the range 1.3 to 1.4. This relatively low value in comparison with that achievable with silver-gelatine emulsions is due to the totally diffuse, matt nature of the platinum-palladium image. From the known coating weights of the sensitized papers the photometric equivalents of platinum and palladium were calculated to be comparable with that of silver.

It is also interesting that Dmax for platinum varies with relative humidity, falling off at the extremes of the range, as is shown in Fig.6. Optimum values of Dmax are obtained within the range 30 - 80% R.H. At very high humidities there is a tendency for the sensitizer solution to diffuse through the paper, with consequent loss of optical density at the surface; at very low humidities, the 'print-out' effect is very incomplete, and subsequent development does not restore the full metal density.

4.4 Colour and Image Quality

Pure platinum images obtained by the present method have a neutral grey tone that only becomes slightly "warmer" at low values of relative humidity (<55% R.H.). In contrast, the palladium sensitizer displays a marked change in image colour with relative humidity, passing from a red-brown at 32% R.H.(in which the high values may be quite dichromatic), through a pleasing purplish-brown at 55% R.H. to a neutral grey-black at 80% and above. Mixtures of the two metals produce corresponding intermediate results, which allow the printer considerable scope in choosing the image colour. The quality of the palladium image is usually perceptibly finer than that of platinum, which has a tendency to display a slightly fibrous character, especially at the extremes of the relative humidity range. This behaviour is however quite dependent on the paper used and is no doubt a consequence of the manner in which the sensitizer penetrates the paper structure.