The trade secrets of the ancient Masters have not come down to us, nor would these to-day serve more than to satisfy our curiosity. The merits of the old tooling are those of the design, and the modern craftsman has at command receipts and processes which from the standpoint of technical results surpass those of the past. The theory of tooling in gold is very simple; the practice is rich in difficulties. Each leather calls for some slight modification of the formula. 
From the craftsman's point of view all leathers are divided into two classes; porous (represented by calf), and non-porous (typified by morocco). 

The former requires some preliminary treatment to fill the pores and make a firm ground for the tooling. This is usually accomplished with a size made from vellum clippings, or a wash of starch paste diluted with vinegar. Morocco having a more solid surface may in most cases be worked as it stands. The first step is making the design. 

It is done on paper with the tools themselves. Bit by bit the pattern is built up, each leaf, each flower calling for a separate impression; each curve may mean the joining of several tools (gouges), each dot is separately impressed. Thus it is seen that the design on the cover of a book may represent many thousand motions by the craftsman. 

The paper bearing the design is then fastened to the leather; the tools are heated, and again the workman goes over the pattern, stamping it through the paper into the leather. When the paper is removed the design is seen tooled in "blind" upon the leather. The surface is then dampened, and the finisher with a camel hair pencil fills the impressions with a size called glaire. This is a solution of albumen in various combinations to suit the nature of the leather. The design is often glaired a second time. When the size is dry, the leather is lightly oiled, and one or more layers of gold leaf are laid on. When the leaf is pressed down with a ball of cotton the pattern is seen through the gold. 
Again the tools are heated to a temperature which varies with the leather and the size of the tool. Again the finisher goes over the design, each tool falling in its former trace. The heated tool coagulates the albumen, which, in its turn, fastens the gold where the tool has struck. The surplus gold leaf, held but lightly by oil, is rubbed off with a bit of flannel. The book is tooled. Such is the philosophy of tooling; very simple in theory, a matter of patience and accuracy of hand and eye; but so perpetually is it complicated with obscure difficulties, that the ideal craftsmen in this kind are few and famous. With these technicalities the connoisseur is not concerned. The question here is; what are the ear marks of fine tooling! At present I put aside matters of design. 

The gold: It should be clear, solid, and unbroken, in appearance a little burnished plate let into the leather, with contours clearly marked. If it is mottled, appears to be burnt in, the craftsman used his tool too hot. If it is broken or imperfect there was not heat enough. If his skill of eye or hand failed him, the impression is "doubled"; he did not strike exactly in the blind impression. The gold should appear to be inlaid; that is to say, it should be sunk below the surface of the leather. Thus it is protected, is permanent and sound. Many a fine piece of early craftsmanship has perished, or is sadly worn, because tooling lay upon the surface. 

But the vice may lie in the other extreme; the leather may be too deeply scored or even burned through to the board. 

All these are faults easy to be marked. But the connoisseur must judge further. He must discern hand tooling from the tread of the stamping press, must distinguish the glittering track of the "roll" from the laborious composition built up of minute tools in patient repetition. This brings one to consider the tools themselves. 

First is the isolated hand tool, the unit, which takes artistic value through its relation with its fellow tool. These are the petits fers; the single leaf, the dot, the flower, or petal of a flower, all of which must fall again and again in its proper place to result in a design. 

Second, there is the composite tool; the complete spray of leaves, or leaves and flower, or arabesque, struck as a whole by hand, or, if large, by the stamping press. These tools resemble in character the fleurons with which the eighteenth century printer graced his pages. Many of them are charming in themselves; but in tooling they are a ready made art, so to speak. The design is not that of the finisher, but that of the engraver. When once their nature is understood they can always be distinguished. 

And, third, of the same nature as the roll. The roll is a wheel on whose surface is engraved a complete running design. This is rolled from point to point by the finisher; and there results a pattern made up of minute elements, but struck as a whole, not piece by piece. Of such, usually, are the "inside borders" of the cataloguer; and of such, sometimes, are his "outside borders" as well. With a little study they can always be detected. Look at the corners where the pattern meets. It very seldom mitres, but over laps, or is clumsily filled in and obscured by a corner ornament. 

Fourth, there is the large composite block, struck by the arming press, named because the block so struck was usually the coat armour of the owner of the book. This must ever be a legitimate embellishment. Books so decorated include many of the choicest specimens of the collector. Arms royal, arms of prelates and warriors, arms of fair bibliophiles, learned or unlearned, virtuous or too fair, were struck thus by the arming press, and will be struck. Such a composition is, in general, too large of face to be impressed by the arm alone. Still in more recent practice coats of arm are built up, piece by piece, where the design is not too intricate and there are no mantles or supporters. It will be seen, I think, that works of the finest sort must always be done with tools of the first class, the petits fers. A little study will enable the bibliophile always to know them. 

Search for the composite tool and roll. If these are absent, one may be sure that the design was wrought bit by bit, was a work of patience, skill, and long labor; unless, indeed, the whole design was machine struck from a solid plate bearing the complete design. But as to this the connoisseur can never be deceived. The machine is not made which in vivacity, variety, brilliancy, and beauty of touch can approach the hand of man. Hand tooling has a sparkle of its own, and a life in it which cannot be mistaken. The tools, falling each in its turn, fall always at a slightly varying angle. 

They are not, and cannot always be held in true perpendicular to the surface of the leather. Thus the work has a thousand minute facets, each with its own angle of reflection; and as the book moves in one's hand, it has ever a new aspect. It retains the emotion of the nerves which wrought it. It sparkles.