August, 1993. Headmaster's office, Hogwarts.

On the last day of the sitting, Master Gainsborough handed a parchment to Dumbledore, and asked him to read it carefully. "Mr. Dumbledore, as I explained when we began your portrait, for me to embed your personality in it you will need to participate in some of my guild's secret magic. You'll understand that we need to protect our trade secrets, so you need to read this carefully and sign it before we continue".

Dumbledore fumbled in a pocket for his glasses and read:

"I swear upon penalty of my magic and my life ..."

"Really Master Gainsborough! Is that necessary?"

"It's the guild's standard vow Headmaster, we've all taken it" came from Dilys Derwent up above Fawkes's perch.

"Oh very well. It seems terribly paranoid though."

"I swear upon penalty of my magic and my life to not ever disclose by word, deed, action or inaction the secrets of the Company of Portraitists which will be revealed to me by Master Humphrey Gainsborough today; that I will allow Master Gainsborough to cast a spell of Binding upon my mind to protect the Company's secrets from retrieval by magical means and to cast the magics necessary to animate my painting after Master Gainsborough has sworn a vow to cast no other spell upon me today; and that I will cast no magic upon Master Gainsborough today."

"That seems very ... thorough. May I see your vow?"

"Of course headmaster. If you are happy with that I shall swear first, then you, and then we can begin."

"I swear upon penalty of my life that I am a Master of the Company of Portraitists; that after Albus Dumbledore has sworn a vow to not reveal the Company's secrets I shall immediately cast upon him a Binding of the mind to protect the Company's secrets from retrieval by magical means; that I will cast no other magic upon Albus Dumbledore today except for that required to animate his portrait; and that in doing so I shall harm no-one."

"Please headmaster, examine the charms and curses upon the vows." Dumbledore placed the tip of his wand on first one, then the other, mumbling rather more diagnostic spells than most customers did while Gainsborough retrieved a blood quill from his bag.

"All seems in order" said the headmaster. "May I check your quill too?"

After some more mumbling, he handed the quill to Gainsborough, who quickly signed his vow before Scourgifying the tip and handing it to Dumbledore. The moment Dumbledore lifted the quill from the page Gainsborough cast his Binding.

"Now, headmaster, for the secret. Magical portraits are animated via a sacrifice. No, please don't interrupt, remember that I swore to harm no-one. We need to perform a ritual in which you sacrifice a tiny amount of cerebral fluid ..."

"I'm afraid I don't know what that is" Dumbledore interrupted.

"It's a liquid that surrounds and permeates every part of your brain. I will extract a very small amount, less than a teaspoon, which your body will naturally regenerate. You will feel a prickling sensation and no more. I will give you a charm to cast on the fluid, linking it to you, I then use it in a potion which I apply to the portrait. Once that is done, when you pass on and your body's cerebral fluid starts to decay your memories will migrate to the fluid preserved in the portrait."

"I would like to examine the charm and potion if I may?"

"Ever the academic Headmaster! Many sitters don't care to, but I had a feeling you would. I brought the text book that our apprentices use. While you look them over may I get started?"

Dumbledore agreed, and Gainsborough produced an improbable amount of equipment from his small bag. A small silver bowl, a short and very thin wand - "clearly designed for precision work" Dumbledore commented - a silver cauldron and tripod, a rough block of amber, an elephant's tusk, and an entire olive tree in a pot.

Gainsborough moved behind Dumbledore. "Could you remove your hat and move your hair aside? Thank you, now for that little prickle." He held the cold silver bowl to the back of Dumbledore's neck, and muttered "Pertundo". Dumbledore could feel a trickle of magic, which stopped the moment he felt something dribble from the point of contact. After a moment the artist reached around him and placed the bowl on the desk, and incanted "Episkey" to heal the puncture.

"Now you need to cast the linking charm."

That was the work of a moment while Gainsborough set up his cauldron. He plucked some olives from the tree - "the fresher they are the better they work, the personality in the portrait can be terribly corrupted if you use stale olives" - and crushed them, extracting the oil which he added to the cauldron. In too went the cerebral fluid, then when the first wisps of steam appeared he added the amber.

"Liqueface liqueface liqueface" Gainsborough chanted. Then as he held the tusk over the potion tapping it with his wand in time with the syllables he mumbled "Memoria aeterna". A pearly dusting of powdered ivory drifted down into the cauldron and a golden glow slowly emerged.

"Now we let it cool just a moment - not too long or it will set, but we don't want to melt the paint."

Gainsborough set a timer on the desk, and rotated his easel into the horizontal. When the last sand fell through the timer he poured the potion onto the painting, and carefully spread a thin coat over the entire surface. It soaked in quickly. "Now to seal it, so no-one else can add memories." Gainsborough quickly applied a light varnish and after washing his brush "Duro" made it set.

"That's the end of the secret Headmaster. With your permission I shall take this back to my studio to apply the final coats of protective varnish and return in two weeks time. Guard that text book with your life!"

Gainsborough disappeared through the fireplace in a flash of green, leaving Dumbledore to consider the implications of "Memoria aeterna" and a certain diary. He had a horrible suspicion about why the painters' guild were so careful with their secret.

Author's notes

Not much happens in this scene, it's really just an exploration of the mechanics of magical portraits and vows. The vows are mutually supporting and are written to only come into effect once both are sworn; the procedure of carefully reading them both and examining the curses on the vows ensures informed consent for otherwise-creepy mind magic - while this is sacrificial magic with attached death-curses, it is still light magic.

Yes, he's related to Thomas Gainsborough.

You may have wondered why Gainsborough swears upon his life, but Dumbledore upon his magic and his life. This is so that the painting can't reveal the secret.

Pertundo, Liqueface, and Memoria aeterna: Latin for "I perforate/pierce", the command "Melt!", and "ever-lasting memory".

Amber is known to preserve that within it; olive trees live for a long time; elephants have excellent memories.

Walburga Black suffered from Stale Olive Syndrome

And the last paragraph ... the Horcrux ritual is indeed a perversion of the portraitists' ritual, involving an unwilling sacrifice of cranial fluid. Murder is not a pre-requisite in this alternate universe, it's just that blowing someone's head off is a quick way of getting at their brains without consent.