3 STRENGTH IMPROVEMENT: COHESION AND ADHESION

3.1 STRENGTH IMPROVEMENT: MATERIALS TESTED

The initial strength of the seven pigments under study, when drawn as films with no binder, varied markedly. Green earth and natural red ochre were found to have very strong initial cohesion. They could be touched or lightly stroked by finger without any mechanical harm, and needed to be scratched by fingernail to be damaged. Raw umber also had good initial cohesion, although less than green earth or red ochre. Ivory black and ultramarine were powdery. They transferred pigment particles when touched but had some resistance to blowing by mouth. Calcium carbonate and chrome yellow were easily blown away by mouth.

Strength was tested on only three pigments, chosen to cover a range of initial cohesion from moderate to very weak: raw umber (moderate), ultramarine (weak), and calcium carbonate (very weak). There was little to be gained from testing pigments that formed paints stronger than one would ever consider consolidating. The pigments were tested on a wood (pine) substrate, in case a porous substrate affected the results. The 0.5% solutions of the four consolidants described in section 2 were misted as a band through the middle of the samples and were applied once on the proper right side and 4 times on the proper left side of the samples. The thickness of the films are given in table 2.

TABLE 2 RESULTS OF THE RESISTANCE OF CONTACT TRANSFERENCE AND RESISTANCE TO BRUSHING ABRASION TEST

3.2 STRENGTH IMPROVEMENT: METHOD OF MEASUREMENT

Two tests were developed: contact transference and brushing abrasion. In the contact test, blotting paper was laid on the surface (black paper for white paint), then a felt cushion, and the sandwich passed through a steel roller mill (used normally for flattening silver). The paper was then inspected for differences in transference, since the treated and untreated areas were side by side. This test would identify not only improvement in surface cohesion but also deficiencies in penetration, since the consolidated layer (skin) would lift away from unconsolidated powder. As noted, each paint had very different initial cohesion before consolidation, so the pressure required to provoke some change also had to be different: raw umber, which was inherently well bonded, required a setting of 3; ultramarine, 12; calcium carbonate, 17 (higher numbers are weaker settings of the spring squeezing the rollers).

The brushing abrasion test used an automatic draw-down device for paint, modified to draw a stiff, coarse-bristle brush across the paint samples at a constant speed and pressure. The number of passes and the weight were adjusted for each paint to discover differences between treatments, since the initial abrasion resistance of each pigment varied so much. The samples were then inspected for differences in the losses caused by the brush, since treated and untreated areas were side by side.

Thus both tests, contact and abrasion, were not absolute comparisons across the three pigments. Absolute comparisons would have required hundreds of tests and yielded trivial results, e.g., the appropriate test for umber would demolish calcium carbonate even at 4 and 10 treatments. These tests measured, for each paint, relative changes in cohesion and adhesion resulting from treatment, given very different initial strengths.

3.3 STRENGTH IMPROVEMENT: RESULTS

The samples are shown in figure 5: at the top are the blotting papers from the contact tests, where transfer of pigment can be observed in some instances; at the bottom are the paints after the brushing abrasion test. The visual assessments are compiled in table 2. Contact tests are given by C, abrasion by A. Two letters (CC or AA) indicate significant improvement to contact or abrasion; one letter means noticeable improvement; and a dash means no noticeable improvement.

Fig. 5. Results of contact transference and brushing abrasion tests for (a) raw umber, (b) calcium carbonate, and (C) ultramarine. The blotting papers at the top are from the contact transference test. At the bottom are the samples on wood after the brushing abrasion test. The center of each sample was treated, once on the left, four times on the right.

Exploratory tests used to set the severity of the brush and contact studies demonstrated that calcium carbonate, the weakest of the unconsolidated pigments chosen in this study, though stronger after 4 treatments with the best consolidant, was still more fragile than unconsolidated raw umber.

AC-33 was not a successful consolidant. On the blotter contact test, the skin stuck to the blotter and peeled off; with the brush abrasion test, the skin flaked off, leaving a powder residue on the wood. One application of 0.5% gelatin or B-72 gave too weak a bond for two of the three pigments. One application of 0.5% methylcellulose gave fair results, being on average as good as 4 applications of B-72. Within the range studied, all pigments improved the most with 4 applications of either 0.5% gelatin or 0.5% methylcellulose.

Some consolidants could weaken a pigment's initial strength: for example, it was found that 4 applications of B-72 on raw umber gave a weaker bond than the unconsolidated raw umber.