How to paint permanent pictures — Reading Companion

The Needle-Proof Test and Drying Realities

Toch introduces a diagnostic method he calls “needle-proof”: a picture is fully dry only when a curved needle cannot puncture its surface. He reports finding paintings years old that still have liquid interiors—a clot of Raw Sienna in a work by Josef Israels remained soft after a decade. This test reveals a central tension in the book: the gap between apparent dryness and actual chemical stability. Toch warns that linseed and poppy oil paints dry from the surface down, forming a wrinkled skin that traps liquid beneath. He advises waiting one to three years before varnishing, especially for heavy impasto or slow-drying pigments. The needle-proof test is not a metaphor; it is a concrete, repeatable procedure that any painter can perform. Toch’s insistence on this test reflects his broader argument that permanence requires patience and scientific understanding, not just artistic intuition.

Varnishes: Solubility as a Virtue

Toch devotes considerable attention to varnishes, ranking them by removability. He prefers mastic and damar because they can be dissolved with turpentine or benzine, allowing safe restoration. He condemns copal oil and amber varnishes for forming hard, insoluble films that force restorers to use strong solvents, risking damage to glazes and delicate tints. The book includes practical notes: mastic may dry unevenly—part glossy, part matte—a defect that can be mitigated with a small addition of lavender oil or linseed oil. Damar behaves differently depending on humidity. These details show Toch’s concern with the unpredictability of materials. He does not simply list varnishes; he evaluates them against the criterion of future reversibility. For Toch, a good varnish is one that can be removed without harming the paint layer beneath—a principle that aligns with modern conservation ethics.

Pigments Under Threat: Sulphur Gases and Restoration

Toch identifies a specific enemy of permanence: sulphur gases in the atmosphere. Pigments such as flake white, chrome yellow, chrome green, Prussian blue, and emerald green are vulnerable to discoloration. He offers a restoration method: wash with weak soap and water, then lightly rub with peroxide of hydrogen. This treatment oxidizes the sulphur-affected compounds and can restore original colors. However, Toch cautions that each case must be handled individually and provides no fixed formula. He also notes that flake white, despite its sensitivity, has superior hiding power—two coats equal four of zinc white. His solution is to seal it with a protective varnish or overlay it with zinc white once dry. The discussion reveals a pragmatic chemist who accepts that some excellent pigments are chemically fragile and that the painter must manage that fragility through technique and timing.

Toch’s book is best read as a period document of applied chemistry, not as a complete painting guide. The excerpts focus on varnishes, drying tests, and pigment vulnerabilities, but the full table of contents shows chapters on tempera, watercolor, painting on metal, and framing. Readers should note that Toch’s recommendations reflect early twentieth-century materials and may not align with current conservation standards. His advocacy for mandatory labeling and his emphasis on reversibility remain relevant. The book rewards those who approach it as a technical treatise rather than a studio handbook.