I searched a little further to see whether gold can get dissolved in ink and whether you can then write with it. I did not find the answer.
Eventually, I came across the following explanation. And, I think, is an intriguing story of an actual event.
I didn't want to withhold the story of György Hevesy from you.
It does show, in theory, there are certainly possibilities. Whether what de Soto did is also practical, I doubt.

As long as you don't do nuclear reactions, atoms are never lost. Gold, which you dissolve in aqua regia, is indeed still present as chloroaurate ions (AuCl4-).
The best story in this context is that of György Hevesy.
During the German invasion of Denmark (WW2), he worked at the Niels Bohr Institute. Hevesy did not want the gold Nobel medals of Max von Laue and James Franck to fall into Nazi hands.
So he dissolved the medals in aqua regia. He left them in a bottle on a shelf with the rest of the chemicals. The solution is brown and quite inconspicuous. After the end of the war, he had the gold precipitated again. This could get done with a reductant, such as metabisulphite, zinc or sulphur dioxide. Afterwards, he sent the gold to the Nobel Foundation, which recast the medals from the recovered gold.

Chapter 30

After a short night, the following morning, everyone was up early.
Felipe and Diego were saddling the horses when the Carvajal brothers arrived at the gate.

After the men had greeted each other, Diego said. "We were just about to go and see you."

Francisco replied. "Against our better judgment, we hoped you would have better news than we have. We have made no progress. Despite your note, don Diego, to decipher what de Soto wrote, the only connection we've discovered to make is that it's all a load of crap."

Diego hadn't expected the brothers to have come to the same conclusions he had. Both men were not dumb. De Soto's trick wasn't something you'd just thought of. "We did come across something."

"You have?" Relief could get scooped from the faces of both brothers.

Diego led the horses to a beam while he said. "Let us go in first. Then I will show you what we have discovered."

"This better not be a joke." Said Estaban.

The horses got tied to a beam. Diego uttered. "I wouldn't dare."

Diego held open the front door and asked the brothers to stand at the dining table.

"Estaban, Francisco. Has Diego told you what he discovered yet?" Alejandro had heard the door and came walking into the room. He was followed closely by Victoria.

Diego spoke calmly. "No, father. I hadn't got to that yet."

Alejandro came to stand by the table. "It is amazing. I couldn't believe my eyes when I saw it."
This made the brothers more curious about what it was Diego had discovered. The old don could barely suppress admiration for his son.
"Well, Diego. Show it to them. I'm sure Estaban and Francisco will be as impressed as I was."

Diego lit a candle, picked up the marble ashtray, tore a few sheets of paper from one of the diaries, and told what had happened the previous evening.
Astonished, the man stared open-mouthed at the remnants and the small amount of gold glistening in the ashtray."

Estaban stuck his finger in the ashtray and picked up some gold on his finger. "This is ingenious. I must give de Soto that credit." He looked at Diego. "And even more clever than you managed to unravel the mystery."

Modestly, a slight smile played around Diego's mouth. "My father deserves all the credit. It was a comment from him that put me on the right track. It made me wonder maybe the ink was the most valuable thing."

Alejandro was interrupting him. "Diego, that's nonsense, and you know it. I was merely saying that de Soto was wasting ink and paper. I confess I am not often proud of you. But at the moment, you give yourself not enough credit. You should be prouder of yourself, for I am. It proves all that book reading and other things you do aren't a total waste of time."
Alejandro gave Diego a firm tap on his shoulder to reinforce his compliment. Diego just managed to grab onto a chair and save himself from falling.

"Your father is right. He may have given you the right idea. It was you who came up with it." Admiration could get heard in Francisco's voice.

Diego was busy putting away the used items back to the right place.
A little shy, Diego said. "As you may have heard, I am interested in various sciences. Few things remain secret in a community like Los Angeles."

"Science is a hobby you share with Zorro, then." Joked Estaban.

"What makes you say that?" asked Alejandro, intrigued, while Diego remained silent for a moment, holding the ashtray and his breath.

"Well, the gliding thing he used to make his entrance this week Zorro made after seeing a design by Leonardo da Vinci, I think. I have seen drawings during my study of something similar. Like Archimedes, da Vinci is considered a genius.

Diego felt uneasy. He had learned in recent days the brothers both had studied at university. Most Los Angeles residents were not interested in where Zorro got his ideas. It was no secret that Diego was a man of science. It had always been one of the distractions to make Diego and Zorro have nothing in common.

The salvation came from Victoria."Da Vinci? Wasn't he some famous painter? He was no a scientist, was he?"

Estaban was ahead of Diego in giving an answer. "Doña Victoria, the man, was a genius. Not only was he a painter. He also was a sculptor, mathematician, physicist, architect, philosopher, chemist, writer, inventor and whatnot else.

Da Vinci was a very versatile man, had many talents, and was far ahead of his time."

"Oh, that sounds rather impressive." Said Victoria politely, showing just enough disinterest to stop Estaban from continuing on the subject.

Francisco said, "That is certainly true. However, your husband's achievement is also definitely something to be impressed about." He changed his attention from Victoria to Diego.
"Don Diego, how much gold do you think is hidden per book?"

"That's hard to say," Diego uttered. "The ratio won't be the same every time.
Certainly, de Soto will have experimented with it at the beginning of what gave the best result.
But there is a simple way to get an estimation. Just wait here."

Victoria offering. "Meanwhile, let me go to the kitchen and fetch us some coffee."

Alejandro insisted on him being the one to get the coffee. "I am the host, and I should have offered it earlier. Please, excuse me while I go ask Maria to bring us some."

Diego walked to the library to get the scale he had already taken from the cave this morning. Victoria was left alone with their guests.
She gestured her hand to the chairs. "Won't you sit down?"

Francisco grabbed a chair, Estaban came to stand beside her making a bow.
"Gracias, doña Victoria. Before we sit, may I apologize to you for what I said on the day my brother got arrested. I had no right to behave so indecently."

"You are forgiven. Of course, you were very concerned about your brother's well-being.
And who am I to judge? I am known for not always being in control of my emotions."

Estaban humbly bowed his head for a second time. "Thank you, doña Victoria. You have a generous heart."

Diego was just in time to see Estaban kiss his wife's hand, and Victoria smiled at the man with all her grace. Estaban pulled a chair back, offering it to his wife. Francisco was sitting on the opposite side of the table, not paying too much attention to the scene.
Diego cleared his throat to make his presence known. Her eyes sought his, smiled, her lips formed his name. "Diego."

Diego placed the scale on the table, as he did with six diaries and the book he had used the night before as a comparison.
"Let us take the average weight of these diaries. We compare them to the weight of this book, and we should get a rough estimate."

Diego weighed the books one by one while Francisco wrote down the weights. Not long after, the sum was complete.
"De Soto had thirty-four books. Based on this average, there are about three ounces of gold per book. Thirty-four times three is one hundred and two," murmured Francisco, writing down his calculation. "Adds up to approximately..., six pounds and over three ounces." Francisco threw the pencil on the table. "And that with the current gold prices...",

Alejandro had come back into the dining room somewhere during the weighting and had started serving coffee to everyone.
He uttered. "They become rather valuable diaries."

Estaban gulped his coffee into his mouth, placing the cup back on the table. "Don Diego, will you come with us to Los Angeles. We will present your findings to the acting alcalde. I think it is best to suggest to alcalde Mendoza that he ask de Soto to give an acceptable explanation of why he did this. If de Soto chooses not to or can't give one, we can indict him for theft of the Spanish crown."

"I come with you." Answered don Alejandro ahead of his son. "As head of one of the most important families in California, forgive me for saying so. I see it as one of my duties to denounce this."

For good measure, Diego sputtered some. He was aware his father was trying to give him an excuse in case his son thought Zorro was needed.
It was the little examples, in moments like this, that made Diego feel grateful nowadays. He began to prefer it over the flash of panic and heart pain afterwards, which he had to suppress for years.
He uttered, "Are you sure you can manage without my help?"

"Of course, son.
I am merely going as a representative of the dons."

Francisco tried to pursue the young don by saying. "Don Diego, surely you want to see the expression on the face of de Soto when he hears you are the one who unravelled his plans?"

"No, thanks. I think it is better to leave it to my father and you. He has more experience in handling these kinds of situations."

"Really, Diego, what do you think de Soto will do? Give you sentence you to get whipped?" Alejandro laughed at his joke. "Diego, he doesn't have that authority anymore." His father remarked, sounding irritated and angrily. "Never mind. You stay here. Francisco, Estaban shall we go?"
Alejandro stormed outside without even looking to see whether the brothers followed him.

Francisco was the one having the most difficulty hiding his awkwardness. Estaban had the courtesy to offer Diego. "We can wait for a while, should you decide to join us. Don't you want to hear what de Soto has to say?"

Diego uttered. "Since you put it that way. Si, I am somewhat curious. I shall join you. Victoria, you don't mind, do you?"

Victoria didn't see any sign Diego wanted to go to the cave. His father had given him enough opportunity, and so she answered. "No, of course not."

Estaban said. "Splendid! Don Diego, we'll go help your father prepare the horses and tell him you will be joining us."

Victoria waited for the brothers to leave the room before she whispered.
"I thought you wanted to make your appearance as Zorro,"

"Completely unnecessary. Zorro can't possibly already know in what way the fraud got committed.
It was my father overlooked. And I also believe Zorro would be better off leaving this to the brothers. They seem more than capable enough to handle this situation."

"Then you go and tell me how de Soto reacted."

.

After he and his father had returned from the pueblo, Diego had told Victoria and Felipe everything about how de Soto had reacted.
De Soto had first boiled with anger, then denied the accusations. Then he had claimed he had no idea who had written the books. Unfortunately for him, there were too many witnesses. Lancers, who knew that their former commander kept diaries.
He had tried to escape, but it had been in vain, for even before de Soto could get close to a horse, the lancers had surrounded him and pointed their guns at him.
Over the past few days, the lancers had gradually admitted that they had not always followed their alcalde's orders blindly.

In the end, Mendoza had decided to follow the advice given by don Alejandro and the Carvajal brothers. He had ordered de Soto's arrest. De Soto was in jail and had to wait for the prisoner transport to get transported to Monterey as soon as possible. Diego had always known de Soto was seeking ambition and wealth. Only in the very beginning, shortly after his arrival, had he hoped de Soto was to change things in Los Angeles for the better. It saddened he had admired once had fallen so deep.

At university, everyone had known about Ignacio de Soto's low social standing. A boy born in a poor family who had by sheer luck got a chance to study and one of the most prestigious universities in Europe.
Yet the young student had managed to rid himself of his low status. He had worked hard to achieve what many considered impossible and had succeeded. Or that was what de Soto had made everyone believe.
Diego had admired the man who managed to work his way up through nothing but hard work. The disappointment he had felt when, after de Soto, his arrival in Los Angeles had been immense.

And the fact that now it had become clear the man had shamelessly enriched himself. Purely out of selfish interests, not taking into account the people from the social class to which he had once belonged. It made Diego feel beaten. Yes, Zorro had fought de Soto for years in a twisted game of cat and mouse. Still, Diego had always been that glimmer of hope that de Soto would one day change his mind and improve for the better. His fixation on the capturing of Zorro as a way to get promoted had always been somewhat amusing.

.

Diego found Victoria in the garden. It was late. She sat on a bench, waving herself some coolness with a van.
After they had accompanied his father to the door. Waving the brothers and alcalde Mendoza off, Diego had gone to the kitchen to thank Maria for the impressive job she had done on such short notice. And to see whether there was some dessert left. Maria always made some more flan, knowing it was his favourite.

Don Alejandro had invited the brothers and alcalde Mendoza for dinner at the hacienda.
With Victoria's help, Maria had managed to put a delicious four-course dinner on the table.
The company had distracted Diego from most of his thoughts on de Soto as far it was possible. Most people in the pueblo cheered after hearing the news de Soto was to get punished for his deeds. Diego felt a second emotion, sadness.

"You were quiet during dinner." Victoria stretched out her arms.

Diego leaned against a tree. "There were too many thoughts wandering through my head."

"De Soto, isn't it? You want to tell me what's going on in your mind?"

"Yeah. You know, I had hopes for him.
Back at the university, I admired him. He was a poor student who worked hard to fulfil his dreams. Even during his years as an alcalde, I always believed that one day he would see all he had to do to get everything he wanted was accept himself. I thought better of him."

"Even with everything, he did to the people in the pueblo and Zorro?"

Victoria had risen to stand closer to him. Diego folded his arms around her.

"He was born and raised as someone who knew how hard life can be. De Soto thought capturing Zorro was the easiest way to get promoted." Diego sighed.

Victoria said in a soft tone, expressing his thoughts.
"And he thought the easiest way to get rich was to commit fraud and force people who have nothing to pay absurd taxes."
Victoria continued speaking in her reassuring tone. "No one can choose where they are born. We can, however, choose what choices we make in life. You chose and try to make the world a little better. De Soto chose to think only of himself." Victoria stroked his cheek.

"Maybe you're right." Diego pushed his head up far enough to kiss her hand. "It was his own decisions that made de Soto the man he has become. He has received the opportunities to be a fair leader."

"I remember what my father told my brothers and me after my mother died. It is the choices we make in life making us to who we are. So make sure you choose your path wisely.
It has made you the man I love. And it makes de Soto have to live with the path he has chosen."

"You're right." Diego pulled Victoria against him. He inhaled her scent. Her closeness alone gave him so much.

"Will you stay with me tonight?" Victoria stared at him, hopefully.

It was a full moon. Usually, on those nights, Zorro went out because that was when criminals were most likely to see their chance.
Diego nodded. He didn't feel like spending half the night in the saddle.

"That is nice."

Together they went inside.

.

.