Dial Planes are of two Sorts; firſt ſuch as are made on the Wall of a Building; or ſecondly, ſuch as are drawn on the Tables of Wood, vulgarly called Dial-Boards.
The firſt Sort, if they are made of Brick-Work, is done by plaistering on the Wall with Lime, Sand and Hair, mixed; this muſt be well drenched with Linſeed Oil, after it is dry, i.e. as long as it will drink any, and then painted with Oil and White-Lead, that it may be durable.
But a better way is to temper the Lime, Sand and Hair with Ox Blood, which will be no great Charge, but of great Advantage; for this Mixture will equal in Time the hardneſs of a Free-Stone, and keep the Surface as free from Injuries of Weather; but you muſt afterwards paint it white. The following Method is ſtill preferable.
How to make an exceeding ſtrong Cement or Plaiſter, with which to form any Dial Plane upon the Side of a Houſe or Wall, which will endure the Weather, not inferior to Stone.
Take Lime and Sand, and temper it with Linſeed Oil to the Conſiſtency of Mortar, or common Plaiſter, and ſpread it upon the Wall to a competent Thickneſs, and it will become as hard as a Stone, and laſt many Ages; and upon this you may deſcribe a Dial, and put on the Lines, Figures and Furniture, as hereafter is directed.
Note, If you are in the Country, remote from any Opportunity of getting Linſeed Oil, you may make it with skimmed Milk, which will be much ſuperior to Plaiſter made with Lime, Hair and Water.
If you are to draw a Dial upon a Stone, the beſt Way is to drench the Stone with Linſeed Oil and White-Lead mixed very thin, till it will drink in no more; then ſhall the Dial you paint upon it, laſt longer, and be the better prepared againſt the Ruins of Time.
Now for Tables or Dial Boards of Wood, they being the moſt common, I ſhall give ſuch Directions for making them, as have been always found moſt profitable and fit for the Purpoſe.
The beſt Woods for this Purpoſe, are the cleareſt Wainſcot, and yellow Fir, provided it is clear of dead turpentine Knots; there is not much Difference between theſe two Woods, as to their Alteration by the Weather, they being both ſubject to ſplit in caſe they are bound, and have not free Liberty to ſhrink with dry Weather, and ſwell with Wet, though as to their laſting, Oak ſeems preferable; though good yellow Fir will laſt the Age of an ordinary Man, if well ſecured, as Things of this Nature ought to be.
In working either of theſe kinds of Woods, the Boards ought firſt to be cut to ſuch a Length as you intend your Dial Board ſhall be, and ſo many of them as may make up the Breadth deſigned, and let them be joined on the Edges, and plained on both Sides, and afterwards ſet to dry.
For it has been obſerved, that though Boards have lain in a Houſe ever ſo long, and are ever ſo dry, yet when they are thus ſhot and plained, they will ſhrink afterwards beyond Belief, if kept dry.
When you think they are dry enough, and will ſhrink no more, let them be ſhot again with good Joints, and let every Joint be ſecured with wooden Dove-Tails, let in a-croſs the Joint in the Backſide; let this be done after the Boards are glued together and well dried: after it has been thus glued, and the Joints are ſufficiently dry, then let the Face of the Board be well plained and tried every Way that it may be both ſmooth and true, and the Edges ſhot true and all of a Thickneſs, as Pannels of Wainſcot are commonly wrought.
The Edges muſt be true and even, that they may fit into the Rabit of a Moulding, put round it, juſt as a Pannel of Wainſcot does in its Frame.
This Method will give Liberty to the Board to ſhrink without tearing; whereas Mouldings that are nailed round the Edges, as the common Way is, do ſo reſtrain the Motion of the Wood, that it cannot ſhrink without tearing; but Boards made this Way will laſt a long Time, without either parting in the Joints or ſplitting in the Wood.
Dials are ſome Times drawn on Planes lined with Copper or Lead, that they may be free from ſplitting or tearing; but a Board (if it be made as before directed) is to be as prefered in many Reſpects. As firſt, In that it is much cheaper. Secondly, Both Lead and Copper will ſwell a little with the Heat of the Sun, and in Time will grow hollow outwards, or become convex inſtead of a perfect Flat, which will much pervert the Truth of its Shadow. And, thirdly, The Colours will be apt to peel from the Metal, and the Dial will by that means be in danger to be ſooner defaced, than if it were painted upon a wooden Plane.
How to make the beſt Glue for gluing the Joints of Boards for Dials.
Take a Quart if Milk (but ſome have prefered Water) that has ſtood ſo long and been skimmed ſo often that no more Cream will ariſe; and when skimmed very clean boil it a little in a leaden Pot, and if any Skum yet ariſe be ſure to take it clean off; then put into the Milk about half a Pound of good Glue cut in ſmall Bits, which will ſoon melt: boil it gently on a ſoft Fire to a good body, but not to be too thick nor too thin; then take it off the Fire and keep it for Uſe.
Note, Care muſt be taken in the boiling that it do not burn to the Sides of the Pot, for that takes away much of the Strength of the Glue: but if it be made with due Care, it binds beyond any other Glue, and it is better able to reſist the Weather, and therfore the fitteſt for gluing Boards for Sun Dials.
Care muſt be taken that your Glue be not made too thin, for if it be, the Wood will ſo drink it up that it will not be of a ſufficient Body to bind the Parts together; on the contrary, if it be too thick, it will not give Way for the Joint to ſhut cloſe enough to be ſtrongly joined; for though it is Glue that makes the Joints ſtick, yet where there is ſo much of it that the Joint cannot cloſe exactly, it will never hold firmly.
Whenever you uſe your Glue take care that it be thoroughly hot, for Glue that is not hot never takes firm hold on the Wood.
You must alſo take great care that the Boards you are to glue have not been touched with Oil or Greaſe; for in ſuch Places the Glue will never take hold, although after a Thing is once glued faſt, no Greaſe nor Oil can hurt it.
The Glue being ready, and the Joints of the Board ſhot true, ſet both the Faces of the Joint cloſe together, and both alſo turned upward; then dip a Bruſh in the Glue and beſmear the Faces of both Joints as quick as poſſible, then clap the two Faces of the Joint together, and ſlide or rub them long ways one upon another two or three times to ſettle them cloſe, and ſo let them ſtand till they are firm and dry.
Mechanick Dialling; Or, the New Art of Shadows – 1756